EC2151 ELECTRIC CIRCUITS AND ELECTRON DEVICES

UNIT I CIRCUIT ANALYSIS TECHNIQUES 12 Kirchoff’s current and voltage laws – series and parallel connection of independent sources – R, L and C – Network Theorems – Thevenin, Superposition, Norton, Maximum power transfer and duality – Star-delta conversion. UNIT II TRANSIENT RESONANCE IN RLC CIRCUITS 12 Basic RL, RC and RLC circuits and their responses to pulse and sinusoidal inputs – frequency response – Parallel and series resonances – Q factor – single tuned and double tuned circuits. UNIT III SEMICONDUCTOR DIODES 12 Review of intrinsic & extrinsic semiconductors – Theory of PN junction diode – Energy band structure – current equation – space charge and diffusion capacitances – effect of temperature and breakdown mechanism – Zener diode and its characteristics. UNIT IV TRANSISTORS 12 Principle of operation of PNP and NPN transistors – study of CE, CB and CC configurations and comparison of their characteristics – Breakdown in transistors – operation and comparison of N-Channel and P-Channel JFET – drain current equation – MOSFET – Enhancement and depletion types – structure and operation – comparison of BJT with MOSFET – thermal effect on MOSFET. UNIT V SPECIAL SEMICONDUCTOR DEVICES

(Qualitative Treatment only) 12 Tunnel diodes – PIN diode, varactor diode – SCR characteristics and two transistor equivalent model – UJT – Diac and Triac – Laser, CCD, Photodiode, Phototransistor, Photoconductive and Photovoltaic cells – LED, LCD.

TOTAL: 60 PERIODS

TEXT BOOKS:

1. Joseph A. Edminister, Mahmood, Nahri, “Electric Circuits” – Shaum series,Tata McGraw Hill, (2001)

2. S. Salivahanan, N. Suresh kumar and A. Vallavanraj, “Electronic Devices and Circuits”,Tata McGraw Hill, 2nd Edition, (2008).

3. David A. Bell, “Electronic Devices and Circuits”, Oxford University Press, 5 th Edition, (2008).

REFERENCES:

1. Robert T. Paynter, “Introducing Electronics Devices and Circuits”, Pearson Education, 7 th

Education, (2006). 2. William H. Hayt, J.V. Jack, E. Kemmebly and steven M. Durbin, “Engineering Circuit

Analysis”,Tata McGraw Hill, 6th Edition, 2002. 3. J. Millman & Halkins, Satyebranta Jit, “Electronic Devices & Circuits”,Tata McGraHill,

2nd Edition, 2008.

UNIT-I CIRCUIT ANALYSIS TECHNIQUES

Ohm’s LawTemperature remaining constant, the potential difference (E) across the ends of a

conductor is proportional to the current (I) flowing through it.Mathematically, V=IR

Kirchhoff's Current Law (KCL)"The algebraic sum of all currents entering and exiting a node must equal zero"

Iin = Iout

Similarly, at any instant the algebraic sum of all the currents at any circuit node is zero.SI = 0

Kirchhoff's Voltage Law (KVL)"The algebraic sum of all voltages in a loop must equal zero"

E = IZ

Similarly, t any instant the algebraic sum of all the voltages around any closed circuit is zero:E - IZ = 0

Series and Parallel Resistor Combinations

There are two basic ways in which to connect more than two circuit components: Series and Parallel.

• For analysis, series resistors/impedances can be replaced by an equivalent resistor/ impedance.

• Parallel resistors/impedances can be replaced by an equivalent resistor/ impedance.

Series ResistanceTwo elements are in series if the current that flows through one must also flow

through the other.

Req is equivalent to the resistor network on the left in the sense that they have the same i-v characteristics.

Parallel ResistanceTwo elements are in parallel if they are connected between (share) the same two

(distinct) end nodes.

Inductors

R1 R2

R1

R2

SERIES Req = R1 + R2 + R3

PARALLEL

Series and parallel inductances

Where, L = Inductance in henrys

Capacitors

Series and Parallel Capacitances

Where, C = Capacitance in farads

Mesh current methodThe Mesh Current Method uses simultaneous equations, Kirchhoff's Voltage Law,

and Ohm's Law to determine unknown currents in a network. It differs from the Branch Current method in that it does not use Kirchhoff's Current Law, and it is usually able to solve a circuit with less unknown variables and less simultaneous equations.

Steps to follow for the .Mesh Current method of analysis:1. Draw mesh currents in loops of circuit, enough to account for all components.2. Label resistor voltage drop polarities based on assumed directions of mesh

currents.3. Write KVL equations for each loop of the circuit, substituting the product IR for

E in each resistor term of the equation. Where two mesh currents intersect through a component, express the current as the algebraic sum of those two mesh currents.

4. Solve for unknown mesh currents (simultaneous equations).5. If any solution is negative, then the assumed current direction is wrong!6. Algebraically add mesh currents to find current in components sharing multiple

mesh currents.7. Solve for voltage drops across all resistors (E=IR).

Node voltage methodThe node voltage method of analysis solves for unknown voltages at circuit nodes

in terms of a system of KCL equations. This analysis looks strange because it involves replacing voltage sources with equivalent current sources.

Node voltage rules:1. Convert voltage sources in series with a resistor to an equivalent current source

with the resistor in parallel.2. Change resistor values to conductance.3. Select a reference node(E0)4. Assign unknown voltages (E1)(E2) ... (EN)to remaining nodes.5. Write a KCL equation for each node 1,2, ... N. The positive coefficient of the first

voltage in the first equation is the sum of conductances connected to the node. Repeat for coefficient of second voltage, second equation, and other equations. These coefficients fall on a diagonal.

6. All other coefficients for all equations are negative, representing conductances between nodes. The first equation, second coefficient is the conductance from node 1 to node 2, the third coefficient is the conductance from node 1 to node 3. Fill in negative coefficients for other equations.

7. The right hand side of the equations is the current source connected to the respective nodes.

8. Solve system of equations for unknown node voltages.

NETWORK THEOREMS:

Thevenin's Theorem:• Any circuit with sources (dependent and/or independent) and resistors can be

replaced by an equivalent circuit containing a single voltage source and a single resistor.

• Thevenin’s theorem implies that we can replace arbitrarily complicated networks with simple networks for purposes of analysis.

Norton's Theorem:Any circuit with voltage sources, resistances (impedances) and open output

terminals can be replaced by a single current source in parallel with single resistance (impedance), where the value of current source is equal to the current passing through the short circuit output terminals and the value of the resistance (impedance) is equal to the resistance seen into the output terminals.

Super Position Theorem:In a linear, lumped element, bilateral electric circuit that is energized by two or

more sources the current in any resistor is equal to the algebraic sum of the separate currents in the resistor when each sources acts separately. While one source is applied, the other sources are replaced by their respective internal resistances.

Super Position Theorem is not valid for power responses. It is applicable only for computing voltage and Current responses.

Maximum Power transfer Theorem:

The maximum Power transferred to a load resistor occurs when it has a value equal to the resistance of the network looking back at it from the load terminals (all sources being replaced by their respective internal resistances). Duality:

Two electrical networks which are governed by the same type of equations are called duality.

For the networks to be duals it is necessary that the variables & elements of one network should also be the duals of variables & elements of other networks.

Method of drawing duality (or) dual network:a) Place a dot in each independent loop of the given network. These dots placed

inside the loops correspond to the independent node in the dual network.b) A dot is placed outside the given network. This corresponds to the reference node

of the dual network.c) All the dots are connected by dotted lines crossing all the branches. The dotted

lines should cross only one branch at a time.d) The dual elements will form the branches connecting the corresponding nodes in

the dual network.

Star-Delta Transformation:

The Star-Delta transformation techniques are useful in solving complex network.

A star network of three resistances RA, RB and RC connected together at common node N can be transformed into a delta network of three resistances RAB, RBC and RCA by the above equations:

In general terms:Rdelta = (sum of Rstar pair products) / (opposite Rstar)

Delta-Star Transformation

A delta network of three impedances RAB, RBC and RCA can be transformed into a star network of three impedances RA, RB and RC connected together at common node N by the following equations:

In general terms:Rstar = (adjacent Rdelta pair product) / (sum of Rdelta)

UNIT-II TRANSIENT RESONANCE IN RLC CIRCUITS

Transient State: If a network contains energy storage elements, with change in excitation, the

current and voltages change from one state to another state is called transient state. The behavior of the voltage or current when it is changed from one state to another state is called transient state.

Transient Time: The time taken for the circuit to change from one steady state to another steady

state is called the transient time.

Natural response: If we consider a circuit containing storage elements which are independent of

sources, the response depends upon the nature of the circuit, it is called natural response.

Transient response: The storage elements deliver their energy to the resistances, hence the response

changes with time, gets saturated after sometime, and is referred to the transient response.

Laplace Transform:The Laplace transform of any time dependent function f(t) is given by F(s).

Where S→A complex frequency given by S=σ + jω

Inverse Laplace Transform:Inverse Laplace transforms permits going back in the reverse direction i.e. from s

domain to time domain.

Order of a System:The order of the system is given by the order of the differential equation

governing the system. If the system is governed by nth order differential equation, than the system is called nth order system.

Q(s) = a0 sn + a1 s n-1+ a2 s n-2 + ……..+an-1 s +an the order of the system is equal to ‘n’.

Initial Value TheoremThe initial value theorem states that if x (t) and x’ (t) both are laplace

transformable, then

Final Value Theorem

The final value theorem states that if x (t) and x’ (t) both are laplace transformable, then

Driving Point impedanceThe ratio of the Laplace transform of the voltage at the port to the laplace

transform of the current at the same port is called driving point impedance.

Transfer Point impedanceThe ratio of the voltage transform at one port to the current transform at the other

port is called transfer point impedance.

Resonant Circuit The circuit that treat a narrow range of frequencies very differently than all other

frequencies are referred to as resonant circuit. The gain of a highly resonant circuit attains a sharp maximum or minimum at its

resonant frequency.

ResonanceResonance is defined as a phenomenon in which applied voltage and resulting

current are in phase.

BandwidthThe Bandwidth is defined as the frequency difference between upper cut-off

frequency (f2) and lower cut-off frequency (f1).

Half Power frequenciesThe upper and lower cut-off frequencies are called the half-power frequencies. At

these frequencies the power from the source is half of the power delivered at the resonant frequency.

SelectivitySelectivity is defined as the ratio of bandwidth to the resonant frequency of

resonant circuit.

Q factorThe quality factor, Q, is the ratio of the reactive power in the inductor or capacitor

to the true power in the resistance in series with the coil or capacitor.

Series Resonance in RLC circuit

In series RLC circuit resonance may be produced by either varying frequency for given constant values of L and C or varying either L and C or both for a given frequency.

At resonance inductive reactance is equal to the capacitive reactance. If f < f0 the current I leads the resultant supply voltage V and so the circuit

behaves as a capacitive circuit at the frequencies which are less than f0. At f = f0, the voltage and current are in phase. The circuit behaves as pure

resistive circuit at the resonant frequency with unit power factor. If f > f0, the current I lags the resultant supply voltage V and so the circuit

behaves as an inductive circuit at the frequencies which are more than f0. At resonance series RLC circuit acts as a voltage amplifier. Series resonance circuit is always driven by a voltage source with very small

internal resistance to maintain high selectivity of the circuit.

Parallel Resonance A parallel circuit is said to be in resonance when applied voltage and resulting

current are in phase that gives unity power factor condition. Parallel resonance is also known as Anti resonance. At anti resonance the parallel resonant circuit acts as current amplifier.

Reactance curvesThe graph of individual reactance versus the frequency is called Reactance Curve.

Types of Tuned circuits

Single tuned circuit Double tuned circuit

Single tuned circuitIn RF circuit design, tuned circuits are generally employed for obtaining

maximum power transfer to the load connected to secondary or for obtaining maximum possible value of secondary voltage.

A single tuned circuit is used for coupling an amplifier and radio receiver circuits.

Double tuned circuit In double tuned circuits, a variable capacitor is used at input as well as output

side. With the help of adjustable capacitive reactance, impedance matching is possible

if the coupling is critical, sufficient or above. It is also possible to adjust phase angle such that impedance at generator side

becomes resistive. The magnitude matching can be achieved by adjusting mutual inductance to the

critical value, which effectively fulfills maximum power transfer condition.

UNIT-III SEMICONDUCTOR DIODES

Energy BandsThe range of energies possessed by an electron in a solid is known as Energy

band.

Classification of Energy Band1. Conduction band2. Forbidden band3. Valence band

Classification of semiconductors1. Intrinsic Semiconductors.2. Extrinsic Semiconductors.

Intrinsic Semiconductors A Semiconductor which is in its extremely pure form is known as an intrinsic

semiconductor. If potential difference is applied across intrinsic semiconductor, the electrons

will move towards the positive terminal while the holes will drift towards the negative terminal.

The total current inside the semiconductor is the sum of currents due to free electrons and holes.

Extrinsic Semiconductors The conductivity can be increased by the addition of a small amount of suitable

metallic impurity. It is also known as impurity semiconductor. The process of adding impurity atoms to the intrinsic semiconductor is called

doping. The purpose of adding impurity is to increase either the number of free electrons

or holes in a semiconductor. Two types of impurity atoms are added to the semiconductor.

Pentavalent impurity atoms - containing five valance electronsTrivalent impurity atoms - containing three valance electrons

Extrinsic semiconductors are classified as,N-type SemiconductorP-type Semiconductor

N-type Semiconductor A small amount of pentavalent impurity is added to a pure semiconductor is

known as N-type Semiconductor. When a pentavalent impurity is added to a pure semiconductor, it displaces

some of its atoms. E.g. ARSENIC (As), ANTIMONY (Sb)

In N-type semiconductor, major part of the current flows due to the movement of Electrons. Therefore electrons in an N-type semiconductor are known as majority carriers and holes as minority carriers.

P-type Semiconductor

A small amount of trivalent impurity is added to a pure semiconductor is known as P-type Semiconductor.

E.g. Gallium (Ga), Indium (In), Aluminium (Al), Boron (B) etc. In P-type semiconductor, major part of the current flows due to the movement

of holes. Therefore holes in a P-type semiconductor are known as majority carriers and electrons as minority carriers.

Conductivity of a semiconductor

Each hole-electron pair created two charge carrying particles is formed. One is negative of mobility µn (free electron) and the other is positive of mobility µp (hole). These particles move in opposite direction in an electric field.

Hence the current density J = σE

Where n , p = magnitude of free electrons & holes respectively.

σ = Conductivity of semiconductor

E= applied electric field

q = Charge of electron or hole.

Hence σ = (nµn + pµp )E

For a pure semiconductor n = p = ni ; where ni – intrinsic semiconductor

CARRIER CONCENTRATIONS IN AN INTRINSIC SEMICONDUCTOR

In order to calculate the conductivity of a semiconductor, it is necessary to know the concentration of free electrons n and the concentration of holes p.

Concentration of Electrons (n)

The number of electrons in the conduction band, n = Nc e- (EC

–EF

)/KT

Where,

Nc = 2 h2

2πmn KT3/2

Concentration of holes (p)

The number of holes in the conduction band, p = Nv e- (EF

–EV

)/KT

Where,

Nv = 2

Energy Gap (EG)

The energy required to break a covalent bond in a semiconductor is known as energy gap. The Energy gap at any temperature is given by, EG = EGO – βT

Drift Current

Drift current is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field applied across the semiconductor material.

Diffusion Current

In a semiconductor material, the charge carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. This movement of charge carriers takes place resulting in a current called diffusion current.

Diffusion current density due to holes, Jp = - q Dp dp/dx A/cm2

Diffusion current density due to electrons, Jn = - q Dn dn/dx A/cm2

Total Current Density

Total current is the sum of drift current and diffusion current.

The total current density for P-type semiconductor Jp = - qpµpE- q Dp dp/dx

The total current density for N-type semiconductor Jn = - qnµnE- q Dn dn/dx

Diffusion Length (L)

The average distance that a charge carrier can diffuse during its lifetime is called as diffusion Length L.

h2

2πmp KT3/2

Theory of PN Junction Diode

When a P-type semiconductor s joined to a N-type semiconductor the contact surface is called PN junction or PN diode.

The voltage across PN junction can be applied in two ways.

(i) Forward biasing(ii) Reverse biasing

The N-type material has high concentration of free electrons and, P-type material has high concentration of holes. At the junction, there is a tendency for the free N-type of diffuse over to the P-side and holes from the P-side to the N-side. This process is called diffusion.

Thus a barrier is set up against further movement of charge carriers. This is called Potential barrier or Junction barrier (VB).The potential barrier is of the order of 0.1V to 0.3 V.

The mobile charges have been depleted in this region. It is known as depletion layer.

ZENER DIODE

A zener diode is a special purpose diode that is operated in reverse-biased conditions. Its operation depends on the zener breakdown phenomenon.

Symbol

Anode Cathode

V-I Characteristics of Zener diode

The operation of zener diode is same as that of ordinary p-n diode order forward biased condition, whereas under reverse biased condition breakdown of the junction occurs.

Breakdown voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and consequently breakdown occurs at lower reverse voltage and further, the breakdown voltage is sharp. The breakdown voltage can be selected with the amount of doping.

The sharp increase in current under breakdown condition is due to the following two mechanisms.

Avalanche breakdown

Zener breakdown

Avalanche breakdown

When doping concentration is less like in ordinary diode then under reverse biased condition a small amount of reverse saturation current flows and is constant as long as the temperature is constant.

When the reverse voltage is increased width of the depletion layer increases at the same time the electrons due to force of attraction by the plates acquire some high velocity and during their motion inside the diode they collide with the electrons in covalent bonds and bring them out.

Due to this multiplication process a large current flows and this kind of breakdown is called Avalanche multiplication or breakdown. Once when this breakdown occurs the diode gets damaged.

Zener breakdown

When doping is heavy then in reverse bias even-before the minority charge carries acquire sufficient velocity the breakdown occurs and is known as Zener breakdown.

In reverse bias under heavy doping condition the width of the depletion layer will be very thin strong electric field exists inside the diode. When reverse voltage increased at once electric field the electrons which are present in the covalent bond are brought due to strong force of attraction. Now, suddenly a large amount of current flows. Nothing but Breakdown occurs. In Zener diode first zener breakdown occurs and later avalanche breakdown.

Applications of Zener diodes

Voltage regulator

Fixing reference voltages in electronic circuits such as power supplies and transistor biasing.

Clippers in wave-shaping circuits.

Square wave generation.

UNIT-IV TRANSISTORS

Introduction of transistors Transistor is a semiconductor device that can amplify electronic signals such as radio and

television signals.

Advantage of the transistor1. Smaller in size2. No filament and no need of power for heating filament3. Low operating voltage4. Higher efficiency

Types of the transistor Unipolar Junction Transistor Bipolar Junction Transistor

Construction of the transistor n-p-n transistor p-n-p transistor

n-p-n transistor It is formed by sand witching p-type semiconductor between two n-type.

p-n-p transistor It is formed by sand witching n-type semiconductor between two p-type.

Terminals for the transistor Emitter Collector Base

Functions of Emitter, Collector & Base:Emitter : To supply majority charge carriers.Collector: To collect majority charge carriers.Base: It passes most of the injected charge carriers to the collector.

Transistor Biasing Applying external voltage to a transistor is called biasing. In order to operate transistor properly as an amplifier, it is necessary to correctly bias the

two pn junctions with external voltages. Depending upon external bias voltage polarities used, the transistor works in one of the

three regions. Active region. Cut-off region. Saturation region.

Sl.No. Region Emitter Base Collector Base Operation of a transistor1 Active Forward biased Reverse biased acts as an amplifier2 Cut-off Forward biased Reverse biased acts as an open switch3 Saturation Forward biased Reverse biased acts as an closed switch

Operation of NPN transistors

Emitter is forward biased & as a result large forward current flows across the emitter junction due to flow of majority carriers.

Injected electrons diffuse into the collector region due to the extremely small thickness of the base.

Collector is reverse bias and creates a strong electrostatic field between base &collector.

Field immediately collects the diffused electrons which enter the collector junction.

Flow of electrons into the base region when confronted with the holes, a few electrons combine & neutralize

Rest of the electrons of the injected electrons diffuse into the collector region and is collected by the collector electrode.

Operation of PNP transistors

Forward bias causes the holes in the P-type emitter to flow towards the base. Reduces the potential barrier at the junction Holes cross the junction & penetrate into the N-region. This constitutes emitter

current IE. Width of the base region is very thin & lightly doped; hence a small amount of the holes

recombine with free electrons of N-regions. This constitutes base current IB & is very small. Rest of the holes drift across the base and enter the collector region and are swept

away by the negative collector electrode. This constitutes base current IC. Current conduction I PNP transistors is by movement of holes. Current conduction in the external circuit is by electrons.

Types of configuration Common Base configuration Common Emitter configuration Common Collector configuration

Common Base configuration Input is connected between emitter & base. Output is connected between collector

&base. Emitter-base junction is forward biased. Collector-base junction is reverse biased. Emitter current IE flows in the input circuit. Collector current IC flows in output

circuit. The ratio of collector current IC, to emitter current IE, is called the Current

amplification factor (α). If there is no input ac signal, then the ratio of IC to IE is called dc alpha (αdc). ac alpha refers to the ratio of change in IC to change in IE. The higher the value of α, better the transistor. α can be increased by making base

thin and lightly doped.

Characteristics of CB configuration

The performance of transistors, when connected in a circuit, may be determined from their characteristic curves that relate different d.c. currents and voltages of a transistor.Such curves are known as Static characteristic curves.

Input Characteristics The curve drawn between Emitter current and Emitter – Base voltage for a given

value of collector-Base voltage is known as input Characteristics. For a given value of VCB,the curve is just like a forward-biased PN junction. With an increase in the value of VCB,it conducts better. This is because of the

effect called early effect or Base width modulation.

Output Characteristics The curve drawn between Collector current and Collector – Base voltage for a

given value of emitter current is known as output Characteristics. The collector current varies with VCB for very low voltage but transistor is never

operated in this region.

Common Emitter configuration Input is connected between base & emitter. Output is connected between collector

& emitter. Emitter-base junction is forward biased. Collector-base junction is reverse biased. Base current IB flows in the input circuit. Collector current IC flows in output

circuit. CE is commonly used because its current, voltage and power gains are quite high

and output to input impedance ratio is moderate. The rate of change in collector current IC, to change in emitter current IE, is called

amplification factor (β).

Input Characteristics The curve drawn between Base current and Base – Emitter voltage for a given

value of collector-emitter voltage is known as input Characteristics. For a given value of VEC,the curve is just like a forward-biased PN junction diode. Input resistance is larger in CE configuration than in CB configuration. This is

because the input current IB increases less rapidly with increase in VBE . An increment in the value of VCE, causes the input current IB to be lower for a

given level of VBE . This is because of the effect called early effect.

Output Characteristics The curve drawn between Collector current IC and Collector – emitter voltage VCE

for a given value of base current IB is known as output Characteristics. Output characteristics in CE configuration have some slope while CB

configuration has almost horizontal characteristics. This indicates that output resistance in case of CE configuration is less than that in CB configuration.

Common Collector configuration

Input is connected between base & collector. Output is connected between collector & emitter.

The Collector forms the terminal common to both the input and output. Base current flows in the input circuit. Emitter current flows in output circuit. With base current IB equal to VCO, the emitter current IE is zero, so no current

flows in the load resistor RL. With increases in input current IB, the transistor passes through the active region

and finally reaches saturation.

Input Characteristics To determine the input Characteristics, VEC is kept at a suitable fixed value. The base-collector voltage VBc is increased in equal steps and the corresponding

increase in IB is noted. This is repeated for different values of VEC.

Breakdown in Transistors Avalanche Multiplication Reach-Through (or) Punch through

Avalanche Multiplication The maximum reverse bias voltage which can be applied before breakdown

between collector and base terminals of the transistor under the condition that the emitter is open-circuited.

It is represented by the symbol BVCBO (for CB configuration). This breakdown voltage is a characteristic of the transistor alone. Breakdown occurs because of the avalanche multiplication of current ICO that

crosses the collector junction. As a result of this multiplication, the current becomes MICO in which M is the

factor by which the original current ICO is multiplied by the avalanche effect. At a high voltage BVCBO, the multiplication factor M becomes infinite and the

region of breakdown is then attained. The current increases abruptly and large changes in current accompanies small

changes in voltage.

Reach-Through (or) Punch through It results from Early effect (i.e.) as a result of increase in VCB and as the doping of

the base is substantially smaller than that of the collector and the penetration of the transition region into the base is larger than into the collector

Since the base is very thin, the transition region spreads completely across the base to reach the emitter junction.

At this point, normal transistor action ceases and the emitter and collector are effectively shorted.

Hence, a large current flows from the emitter to collector. This is called Reach-through.

Field Effect Transistor (FET)

FET is a semiconductor device which depends for its operation on the control of current by an electric field.

The output characteristics of FET are controlled by Input voltage and not by the Input current.

So, it is also known as voltage-controlled device.

Features of FETThe FET has several advantages over the conventional transistor.

Its operation depends upon the flow of majority carrier only. So, it is called as Unipolar device.

It is relatively immune to radiation. It exhibits a high input resistance, typically many mega ohms. It is less noisy than a tube of a Bipolar Transistor. It exhibits no offset voltage at zero Drain current. It has thermal stability.

Types of FET Junction Field Effect Transistor (JFET) Metal Oxide Field Effect Transistor (MOSFET) (or)

Insulated Gate Field Effect Transistor (IGFET)

Construction of JFET JFET is a three terminal semiconductor device in which current conduction is by

one type of carrier either Electrons or holes. The JFET consists of a P-type or N-type silicon bar. The bar is made up of N-type material which is known as N-channel JFET and if

the bar is made up of P-type material, it is known as P channel JFET. The current in FET is carried by the majority carriers. One end of the channel is called the source and the other is called the drain.

Operation of JFETFET works under the three conditions. When VGG applied and VDD=0 When VDS applied and VGG=0 When VDD applied and VGG is applied.Where, VGG – Gate supply voltage. VDS– Drain Source voltage. VDD– Drain supply voltage.

Characteristics of JFET A family of curves that relate the current and voltage are known as

characteristics curve. There are the two important characteristics of a JFET.

Transfer characteristics Drain characteristics

Characteristics Parameters of JFETThe parameters of JFET are

Transconductance Drain resistance Drain conductance Amplification factor

Metal Oxide Semiconductor FET (MOSFET) MOSFET is a three terminal device. Those terminals are source, gate and drain. The gate of a MOSFET is insulated from the channel. Because of this, the MOSFET is also known as an IGFET (Insulated gate FET). The MOSFET is a second category of FET. The MOSFET differs from the JFET is that it has no pn junction structure; instead

the gate of the MOSFET is insulated from the channel by a silicon dioxide layer.

Types of MOSFET Depletion – type MOSFET Enhance – type MOSFET

Construction of MOSFET Two highly doped n regions are diffused into a lightly doped p type substrate. These two highly doped regions are represents source and drain. In some cases

substrate is internally connected to the source terminal. The source and drain terminals are connected through metallic contacts the n-

doped regions linked by an n-channel. The gate is also connected to a metal contact surface but remains insulted from

the n-Channel by a very thin layer of dielectric material, Silicon Dioxide. This layer act as one parallel plate capacitor. Thus, there is no direct electrical connection between the gate terminal and the

channel of a MOSFET increasing the input impedance of the device.

Characteristics of MOSFETThe different characteristics of a D-MOSFET are

Drain characteristics Transfer characteristics

UNIT-V SPECIAL SEMICONDUCTOR DEVICES

Tunnel Diodes When the impurity concentration is of the order of one part to 103 parts then

tunnel diode is formed. This diode has negative resistance region. Due to which it is used as an oscillator. This diode is uses the tunneling phenomenon.

TunnelingThe process that an electron from n-side of a pn diode directly penetrates through

the junction into the p-side of diode is called tunneling. It is a quantum –mechanical behaviors.

Operation When a tunnel diode is under unbiased condition then there will not transfer of

electrons from n-side to p-side hence the net current will be zero. When the diode is reverse biased under this condition the electrons from n-side

are attracted by the positive plate and hence move away from the junction. As a result the energy level in the n-side decreases when compared to the

unbiased state. Now, there will be some empty state in valence band of p-side quite opposite to

the empty conduction band. Hence tunneling takes place from p to n-side. As reverse bias is increased this current increase.

Applications Tunnel diode is used as Ultra-high speed switch. Used in relaxation oscillator. Used as an amplifier. Used as logic memory storage device. Used as microwave oscillator.

Advantages High speed operation Ease of operation Low noise Low cost Low power

Disadvantages It is two terminal device, there is no isolation between the input and output circuit. Voltage range over which it can be operated is 1 V or less.

PIN Diode

It has highly improved switching time in comparison with a PN diode. PIN diodes are used in microwave switches. In PIN diode high resistivity intrinsic layer is sandwiched between the P and N

regions. This results in improved switching time. Quite often instead of I-region we actually use either a high resistivity P-region is

called π region and the high resistivity N-region is called γ region. The I-region has typically resistivity of 10 Ωm.

Applications of PIN Diode Used as pulse and phase shifter. Used as SPST and MPST switches. Used in amplitude modulation. Used as photo detectors in fiber optic systems. Used as T-R switch. Used as attenuator and duplexer.

Varactor diode Varactor diode is a specially manufactured reverse biased PN junction diode with

a suitable impurity concentration profile. It is also called as varicap or voltacap. It is used as a variable reactance capacitance.

Characteristics of Varactor diode The diode conducts normally in the forward direction. At relatively low voltage the reverse current saturates and then remains constant. It is rising rapidly at avalanche point. At the saturation point the maximum junction capacitance is obtained and a point

just above avalanche the minimum junction capacitance is obtained. Therefore there are two conditions which are limiting the reverse voltage swing

and the capacitance variation.

Applications Used as a tuning device in receivers. It is used in High frequency. It is used in adjustable band-pass filter It is used in FM modulation. It is used in automatic frequency control devices. It is used in parametric amplifier.

SCR SCR consist of four semiconductor layers forming a PNPN structure. It has three PN junctions namely J1, J2, J3. There are three terminals called anode (A), cathode (K) and the gate (G). The anode terminal is taken out from P1 layer, and the gate (G) terminal from the

P2 layer. It conducts the current in forward direction only.Operation of SCR

SCR is forward bias with a small voltage, it is in ‘OFF’ and no current flows through the SCR.

The applied forward voltage is increased, a certain critical voltage called forward break over voltage (VBO).It reaches at the junction J2 breakdown. At this case the SCR switched ‘ON’ position.

If the SCR is reverse bias, the junction J1 and J3 are reverse bias and junction J2 is forward bias.

It has found that most of the voltage will drop across junction J1 only. When the applied reverse voltage is small, the SCR is OFF, and there is no

current flow through the device.

SCR characteristics It is the relationship between the anode –cathode voltage and anode current at

different gate current. Two types of V-I characteristics

Forward Characteristics Reverse Characteristics

Forward Characteristics It is the current drawn between anode-cathode voltage (VAK) and anode current

(IA) at different gate current. Adjust the gate current to zero value by keeping the switch open. Increase the applied voltage across the SCR in small suitable steps at each step. Note the anode current & plot the graph.

Reverse Characteristics The reverse characteristic is obtained by reversing the connections of the d.c.

supplies VAA and VGG . Adjust the gate current to any suitable value. Increase the reverse applied voltage in suitable steps. Note the anode current for each steps. Now we plot a graph with anode current and anode cathode voltage.

Turning ON (Triggering) SCRThe SCR can be turned ON, from OFF position by anyone of the following methods.

Gate triggering Forward break over voltage Light triggering Rate-effect

LatchingOnce the SCR is turned ON, it starts to conduct and remains in conduction state

even when the gate signal is removed. This ability of the SCR to remain conducting, even when the gate signal is removed, is known as latching.

Turning OFF

One of the following methods is applied to turn OFF the SCR. Reversing polarity of anode-to-cathode voltage called as Gate turn OFF switch

(GTO). The second method is anode current interruption. Changing anode current by

means of momentarily series or parallel switching arrangement. Third method is forced commutation. In this, the current through SCR is reduced

below the holding current.

Applications of SCR Power control device Relay control Regulated power supplies Static switches Motor control Battery charges Heater controls Phase controls For speed control of DC shunt motor

Advantages of SCR SCR controls large current in the load by means of a small gate current. SCR size is very compact. Switching speed is high.

UniJunction Transistor (UJT) UniJunction transistor is a three terminal semiconductor device consisting of only

one PN junction. It differs from ordinary PN diode in the sense that it has three terminals namely

Emitter, Base1 and Base 2. The behavior of UJT differs from other transistors like BJT and FET in the sense

that it has no ability to amplify. However, it has ability to control large ac power with a small signal. It also exhibits a negative resistance characteristic which allows it to be used as an

oscillator.

Applications of UJT Non sinusoidal oscillator Timing circuits Saw tooth generators Triggering device for SCR and TRIAC Switching circuits Voltage regulated supply

Diac (Diode A.C. switch)

A DIAC is two terminal semiconductor device and three layer bidirectional device, which can be switched form of it’s OFF to ON state for either negative or positive polarity of applied voltage.

The two leads are connected to p-region of silicon separated by an n-region. It consists of two 4-layer diodes connected in parallel in opposite direction.

The diodes are P1N1P2N2 and P2N1P1N3. It has two main terminals namely Main terminal 1 and Main terminal 2.

Applications of DIAC Temperature control Triggering of TRIAC Light dimming circuits Motor speed control

Triac (Triode A.C. switch) TRIAC is a three terminal semiconductor switching device which can conduct in

either forward or reverse direction. The TRIAC is the combination of two SCR’s connected in parallel but in opposite

direction. The anode of one SCR is connected to the cathode of another SCR. The gates are connected together. It consists of two four layer switches in parallel and the switches are P1N1P2N2 and

P2N1P1N4. The TRIAC has two main terminals namely main terminal1 and main terminal2

and one Gate terminal.

Applications of TRIAC Heater control Phase control Light dimming control Static switch to turn a.c. power ON and OFF. Speed control of motor.

Light Activated SCR (LASCR) LASCR is similar to that of a SCR except the light triggering. It has a window and lens which focuses light on the gate junction area. It can be triggered ON by a light input on the gate area, but does not turn OFF,

when light source is removed. The LASCR acts like a latch. To reduce the holding current, it can be turned OFF. Depending on its size a LASCR is capable of handling larger amount of current. It can be handled by a photo transistor or a photo diode.

Applications of LASCR Optical light controls

Phase control In relays Motor control

LASER DIODE The term Laser comes from the acronym for light amplification for

stimulated emission of radiation. The Laser medium can be a gas, liquid, amorphous solid or

semiconductor. Two commonly used Laser structure

PN homojunction laser Double hetrostructure laser

Laser Action The light traveling through a semiconductor, then a single photon is able to

generate an identical second photon. This photon multiplication is the key physical mechanism of lasing. The carrier inversion is the first requirement of lasing. It is achieved at the PN junction by providing the conduction bandwidth electrons

from the N-doped side and the Valence band with the holes from the P-doped side.

The photon energy is given by the band gap, which depends on the semiconductor material. The optical feedback and the confinement of photon in an optical resonator are the second basic requirement of lasing.

Photodiode It is a light sensitivity device used to convert light signal into electrical signal. It is also called Photo detector. The light energy fall on the junction through lens, when, the PN photodiode

junction is reverse bias. The hole-electrons pairs are created. The movement of the hole-electron pairs in a properly connected circuit results in

current flows. The current is proportional to the intensity of light and the frequency of the light

falling on the junction of the photo diode. It is used in demodulator, encodes and light detectors systems.

Phototransistor The photo transistor is a light detector. It combines a photodiode and phototransistor. The phototransistor cannot be directly used in control applications. Because, it

produces a very low current. Before applying to control circuit the current should be amplified. A lens focuses the energy on the base-collector junction. It has three terminal, but only two leads are generally used (emitter and collector). The base current is supplied by the current created by the light falling on the

Base-collector photodiode junction.

In phototransistors, the current is dependent mainly on the intensity of light entering into the lens and the voltage applied to the external circuit.

Photoconductive sensors Photoconductive sensor is also called as Light Depending Resistor (LDR). It is made of thin layer of semiconductor material (cadmium sulfide). There is no light falls on the sensor the resistance is very high and the current is

low. Hence, the voltage drop across R is high. It is used in control circuits to control

the current.

Photovoltaic sensors It is a light-sensitive semiconductor device, and it produces a voltage, when the

voltage increases and the intensity of light falling on the semiconductor junction of this photovoltaic cell increases.

It consists of a piece of semiconductor material (silicon or germanium). The photovoltaic cells are produced more power, as in solar cells. These are

called photovoltaic devices. It is used in light meters.

LIGHT EMITTING DIODE (LED) An LED is a semiconductor p-n junction diode which converts electrical energy to

light energy under forward biasing. It emits light in both visible and IR region. The amount of light output is directly proportional to the forward current. LED structure can be divided into two categories.

Surface - emitting LED Edge - emitting LED

Surface emitting LED’s emit light perpendicular to the PN junction plane. Edge-emitting LED emits light parallel to the PN in the plane.

Principle and Working Injection luminescence is the principle used in LED’s. When LED is forward biased, the majority charge carriers moves from p to n and

similarly from n to p region and becomes excess minority carriers. These excess minority carriers diffuse through the junction and recombines with

the majority carriers in n and p region respectively to produce light. The light thus produced is emitted from the p-n junction of the diode.

Advantages of LED They are smaller in size. Its cost is very low. It has long life time. It operates LED’s are available in different colours at low cost. even at very low voltage. Response time of LED is very fast in the order of 10 9 seconds.

Its intensity can be controlled easily. It can be operated at a wide range of temperature (0-70˚) C.

Applications of LED Used for numeric display in pocket calculators. Used for applying input power to lasers. Used for entering information into optical computer memories Used for solid video displays. Used in image sensing circuits.

Liquid Crystal Display (LCD) Liquid crystal display is not a semiconductor device as LED. LCD’s display the light, it doesn’t radiate light energy. Therefore, LCD’s require an external (or) internal source of light so that it can

either transmit (or) reflect the incident light. LCD is a passive type display device used to display alpha numeric character and

is seven segment display, watches calculators etc., in which the digits are displayed by the transmission (or) deflection of the incident light, with very low power consumption.

Molecules in ordinary liquids have random orientation but in a liquid crystal they are oriented in a definite crystal pattern.

Types of LCD’so Dynamic Scattering Displays.o Twisted nematic display (or) Field effect display

Advantages of LCD Low power is required Good contrast Low cost

Disadvantages of LCD Speed of operation is slow LCD occupy a large area LCD life span is quite small, when used on d.c. Therefore, they are used with a.c.

suppliers.

Applications of LCD Used as numerical counters for counting production items. Analog quantities can also be displayed as a number on a suitable device. (e.g.)

Digital multimeter. Used for solid state video displays. Used for image sensing circuits. Used for numerical display in pocket calculators.

EC2151 ELECTRIC CIRCUITS AND ELECTRON DEVICES

UNIT – I CIRCUIT ANALYSIS TECHNIQUES

1. What is charge?The charge is an electrical property of the atomic particles of which matter consists. The unit of charge is the coulomb.

2. Define current?The flow of free electrons in a metal is called electric current. The unit of current is the ampere. Current (I) = Q/t, Where Q is total charge transferred & T is time required for transfer of charge.

3. What is voltage?The potential difference between two points in an electric circuit called voltage. The unit of voltage is volt. It is represented by V OR v.

Voltage = W/Q = workdone/Charge

4. Define power.The rate of doing work of electrical energy or energy supplied per unit time is called the power. The power denoted by either P of p. It is measured in Watts. (W).

Power = work done in electric circuit/TimeP = dw/dt = dw/dq . dq/dt

P = VI

5. What is network?Interconnection of two or more simple circuit elements is called an electric network.

6. Distinguish between a branch and a node of a circuit.A part of the network which connects the various points of the network with one another is called a branch. A point at which two or more elements are jointed together is called node.

7. Distinguish between a mesh and a Loop of a circuit.A mesh is a loop that does not contain other loops. All meshed are loops. But all loops are not meshes. A loop is any closed path of branches.

8. Define active and passive elements.The sources of energy are called active element.Example: voltage source, current source.The element which stores or dissipates energy is called passive element.Example: Resistor, Inductor, Capacitor.

9. Define unilateral and bilateral elements.In unilateral element, voltage – current relation is not same for both the direction.Example: Diode, Transistors.In bilateral element, voltage – current relation is same for both the direction.Example: Resistor

10. Define linear and non-linear elements.If the element obeys superposition principle, then it is said to be linear elements.Example: Resistor.If the given network is not obeying superposition principle then it is said to be non linear elements.Example: Transistor, Diode.

11. Define Lumped and distributed elements.Physically separable elements are called Lumped element.Example : Resistor, Capacitor, Inductor.A distributed element is one which is not separable for electrical purpose.Example : Transmission line has distributor resistance, capacitance and inductance.

12. How are the electrical energy sources classified?The electrical energy sources are classified into:1. Ideal voltage source 2. Ideal current source.

13. Define an ideal voltage source.The voltage generated by the source does not vary with any circuit quantity. It is only a function of time. Such a source is called an ideal voltage source.

14. Define an ideal current source.The current generated by the source does not vary with any circuit quantity.It is only a function of time. Such a source is called as an ideal current source.

15. What are independent source?Independent sources are those in which, voltage and current are independent and are not affected by other part of the circuit.

16. What are dependent sources?

Dependent sources are those in which source voltage or current is not fixed, but is dependent on the voltage or current existing at some other location in the circuit.

17. What are the different types of dependent or controlled sources?1. Voltage Controlled Voltage Sources (VCVS)2. Current Controlled Voltage Sources (CCVS)3. Voltage Controlled Current Sources (VCCS)4. Current Controlled Current Sources (CCCS)

18. What is resistance?It is the property of a substance which opposes the flow of current through it. The resistance of element is denoted by the symbol “R”. It is measured in Ohms.

R = PL / A Ω

19. Write down the V-I relationship of circuit elements.

Circuit Element

Voltage Current Power

Resistance V = iR i = V / R P = vi

Inductance V = L di /dt i = 1/L ∫ vdt P = Li di / dt

Capacitance V = 1/c ∫ idt i = C dv / dt P = CV dv / dt

20. What is average value?It is defined as area under one complete cycle to period.The average value of the sine wave is the total area under the half-cycle curve divided by the distance of the curve.

Area under one complete cycleAverage value = _____________________________

Period

21. Define R.M.S. value.The r.m.s value may be determined by taking the mean of the squares of the instantaneous value of current over one complete cycle.

(Area under hatched line) ²

RMS = ______________________Period

22. Define form factor.The ratio of RMS value to the average value is called the Form factor.

RMS valueForm factor (Kf) = ______________

Average Value

23. Define peak factor.Peak factor is defined as the ratio of the maximum value to the rms value.

Maximum value

Peak Factor (Kp) = ________________ RMS value

24. Define Ohm’s law.The current flowing through the electric circuit is directly proportional to the potential difference across the circuit and inversely proportional to the resistance of the circuit, provided the temperature remains constant.

25. Define Kirchoff’s current law.Kirchhoff’s current law states that in a node, sum of entering current is equal sum of leaving current.

∑ I at junction point = 0

26. Define Kirchoff’s voltage law.Kirchhoff’s Voltage Law (KVL) states that the algebraic sum of the voltages around any closed path is zero. Around a closed path ∑ V= 0.

27. Two resistances with equal value of “R” are connected in series and parallel. What is the equivalent resistance?

Resistance in series Req = R1+R2

R1R2 Resistance in parallel Req = __________

R1+R2 28. Two inductors with equal value of “L” are connected in series and

parallel what is the equivalent inductance?

Inductance in series Leq = L1 + L2

L1L2 Inductance in parallel Leq = _______

L1+L2

29. Two capacitors with equal value of “C” are connected in series and parallel. What is the equivalent capacitance?

C1C2 Capacitance in series Ceq = __________

C1+C2

Capacitance in parallel Ceq = C1 + C2

30. Write down the formula for a star connected network is converted into a delta network?

R R + R R + R R A B B C C A

R = ____________________________ AB R

CR R + R R + R R A B B C C A

R = ____________________________ BC R

AR R + R R + R R A B B C C A

R = ____________________________ CA R

B

31. Write down the formula for a delta connected network is converted into a star network?

R * R AB CA

R = ____________________________ AB

R + R + R AB BC CA

R * R AB BC

R = _____________________________ B

∑R AB

R * R BC CA

R = _____________________________ B

∑R AB

Where ∑ R = R + R + RAB AB BC CA

32. Write few advantages of sinusoid waveform?1. The response of the sinusoidal input for second order system is sinusoidal.2. The wave form can be written in terms of sinusoidal function according to

fourier theorem.3. The derivatives and integral also sinusoidal.4. Easy for analyses.5. Easy to generate.6. More useful in power industry.

33. Distinguish between a cycle, time periods and frequency. One complete set of positive and negative instantaneous values of the voltage

or current is called cycle.

The time taken by an alternating quantity to complete one cycle is called time period (T).

2πTime period (T) = _________

W The number of cycle that a alternating quantity completed per second is known

as frequency. It is measured in HZ.1

Frequency (f) = ________T

34. What is instantaneous value? The value of an alternating current, at any particular moment is called its

instantaneous value.

35. What are peak value and peak to peak value? The peak value of the sine wave during positive or negative half only.

The sum of positive and negative value is called a peak to peak value. The peak to peak value of a sinusoidal alternating voltage is equal to two

times the peak value.

36. What is average value? The average value of the sine wave is the total area under the half-cycle curve divided by the distance of the curve.

1 TV av = ___ ∫ V (t) dt T 0

37. Define effective value or RMS value of a sinusoidal voltage. The R.M.S value may be determined by taking the mean of the squares of the Instantaneous value of current over one complete cycle. This is often known as the effective value.

V rms = ____________ √ 1/T T 2

∫ [V (t) dt 0

38. Define phasor. Graphical representation of phasors V = Vm <φ and I=I < - θ known phasor.

39. Define phase angle. The angle between voltage and current is called phase angle. It is denoted by

φ.

40. What is impedance? The ratio of the phasor voltage to the phasor current is called impedance. It is denoted as Z and it is measured in Ohms.

VImpedance (Z) = ____

I 41. Obtain the equivalent impedance and reactances for series and parallel connections.

Impedance in series Z eq = Z1 + Z2

Impedance in parallel Z eq = Z1 Z2

________ Z1 + Z2

Reactances in series j X eq = j (X1 + X2)

X1 X2

Reactances in parallel j X eq = j ____________ X1 + X2

41. Define admittance. The reciprocal of impedance is called admittance. It is denoted as Y and it is

measured in Siemens (S).

1 1Admittance (Y) = ___ = ___

Z V

42. Obtain the equivalent admittance for series and parallel connections.

Y1 Y2

Admittance in series Y eq = ____________ Y1 Y2

Admittance in parallelY eq = Y1 Y2

43. Obtain the equivalent conductance and susceptance for series and parallel connections.

G1 G2

Conductance in series G eq = _________G1 + G2

Conductance in parallel G eq = G1 + G2

Susceptance in series B1 B2 jB eq = __________

G1 G2

Susceptance in parallel jB eq = j(B1 B2)

44. Define phasor diagram.The phasor diagram is a name given to a sketch in the complex plane showing the

relationships of the phasor voltages and phasor currents throughout a specific circuit.

45. What is instantaneous power? The power at any instant of time is known as instantaneous power.

P (t) = v(t) . I (t)

46. What is average power? The average of the instantaneous power over one period is called average power.

Average power is also defined as the product of voltage and current.

47. What is apparent power? The product of V rms and I rms is known as the apparent power (s).

Apparent power (S) = V Eff I Eff VA

48. Define power factor. The ratio of the average power to the apparent power is called the power

factor.

Average powerPower Factor = _________________

Apparent Power

49. What is power triangle ? A commonly employed graphical representation of complex power is known

as the power triangle.

50. Define complex power

The product of the rms voltage phasor and the complex conjugate of the rms current phasor is known as complex power. It is denoted as S and it is measured in volt-amperes (VA)

The complex power is S = _1__ vi*2

51. What is reactive power? It is defined as product of the applied voltage and the reactive component of

the current. It is also called as imaginary component of the apparent power. It is represented by “Q” and it is measured in unit volt- ampere reactive (VAR).

Q = V Eff I Eff sin φ VAR

52 What is the equation for determining the number of independent loops in mesh current method?

L = b-n +1Where L = number of loops

B = number of branchesN = number of nodes.

52. On which law is the mesh analysis based?Mesh analysis is based on Kirchoff’s voltage law.

53. On which law is the nodal analysis based? Nodal analysis is based on Kirchoff’s current law and Ohm’s law.

54. What is mesh analysis? Mesh analysis is one of the basic techniques used for finding current flowing

through the loop in a network. Mesh analysis is applicable if the given network contains voltage sources. If

therer exist current sources in a circuit, then it should be converted into equivalent voltage sources.

55. What is nodal analysis? Nodal analysis is one of the basic techniques used to finding solution for voltage

drop across the nodes in a given circuit.Nodal analysis is applicable if the given network contains current sources. If

there exists voltage sources in the given circuit, then it can to be converted into equivalent current sources.

56. When do we go for supermesh analysis. If the branches in the network has a current source, then it is slightly difficult to

apply mesh analysis.One way to over come this difficulty is by applying the supermesh technique.In this case we have to choose supermesh.A supermesh is constituted by two adjacent loops that have common current

source.

57. When do we, go for supernode analysis.

If the branches in the network has a voltage source, then it is slightly difficult to apply nodal analysis.

One way to overcome this difficulty is by applying the supernode technique.In this case, we have to choose super node.A supernode is constituted by two adjacent node that have common voltage

source.

58. State superposition theorem.Any electric circuit (linear, lumped, bilateral), is energeied by two or more

sources, the response in any element in the network is equal to the algebraic sum of the responses caused by individual sources acting separately.

59. State Thevenin’s Theorem.A complex network having linear, bilateral, lumped elements with open

circuited output terminals can be reduced by a simple circuit consisting of a single voltage source in series with a impedance.

60. State Norton’s theorem.Any electrical network (linear, lumped, bilateral) with short circuited

terminals can be reduced by a simple circuit consisting of a single current source in parallel with a Thevenin’s equivalent resistance.

61. State Maximum power transfer theorem.Power transferred from source to load will be maximum, when source

resistance is equal to load resistance looking back from its load terminals.

62. Define duality.Two electrical network which are governed by the same type of equations

are called duality.

UNIT – II TRANSIENT RESONANCE IN RLC CIRCUITS

1. What is transient state?If a network contains energy storage elements, with change in excitation,

the current and voltages change from one state to other state. The behaviour of the voltage or current when it is changed from one state to another state is called transient state.

2. What is transient time?The time taken for the circuit to change from one steady state to another

steady state is called transient time.

3. What is natural response?If we consider a circuit containing storage elements which are independent

of sources, the response depends upon the nature of the circuit, it is called natural response.

4. What is transient response?The storage elements deliver their energy to the resistances, hence the

response changes with time, gets saturated after sometime, and is referred to the transient response.

5. Define Laplace transform function.The laplace transform of any time dependent function f (t) is given by

F(s).Where S A complex frequency given by S = σ + jw ∞

F (S) = L [F (T) ] = ∫ F (t) e -st dt 0

6. What is inverse Laplace transform?Inverse Laplace Transform permits going back in the reverse direction i.e.

from s domain to time domain.

1 σ1 + j∞ L-1 [F(s)] = f (t) = _________ ∫ F (s) e st ds

2Лj σ1 - j∞

7. Define time constant or RL Circuit.The time taken to reach 63.2% of final value in a RL Circuit is called the

time constant of RL circuit.Time constant (t) = L / R

8. Define time constant of RC Circuit.The time taken to reach 36.8% of initial current in an RC circuit is called

the time constant of RC circuit.

Time constant (t) = RC.

9. What is meant by natural frequency?

If the damping is made zero then the response oscillates with natural frequency without any opposition, such a frequency is called natural frequency of oscillations, denoted as ώ

n.10. Define damping ratio.

It is the ratio of actual resistance (R) in the circuit to the critical resistance (R cr). It is denoted by greek letter Zeta (ξ).

R Rξ = ____ = _____ √ C / L

R cr 2 2

11. Define initial value theorem.The initial value theorem states that if f (t) and f’ (t) both are laplace

transformable, ThenLim f (t) = lim s F (s)T 0 s ∞

12. Define final value theorem.The final value theorem states that, if f (t) and f ‘ (t) both are laplace

transformable, thenLim f (t) = lim s F (s)T ∞ s 0

13. What is driving point impedence ?The ratio of the Laplace transform of the voltage at the point to the laplace

transform of the current at the same port is called driving point impedance.The driving point impedance of the network is define as

V(s)Z (S) = _____________

I (s)

14. What is transfer point impedance? It is defined as the ratio of voltage transform at one port to the current

transform at the other port. It is defined by V2 (s)

Z21 (s) = ____________ I 1 (s)

and

V1 (s)Z 12 (s)= ____________

I2 (s)

15. Define network function.A network function N (S) is defined as the ratio of the complex amplitude

of an excponential output P (S) to the complex amplitude of an exponential input Q (S).

16. Define pole and zero.The network function N (S) will become infinite. Hence the roots of

denominator polynomial P1, P2, P3,……Pm are called poles of network function.The network function N(S) will become zero. Hence the roots of

numerator polynomial Z1, Z2, Z3…………..Zn. are called zeros of network function.

17. Define resonant circuit.The circuit that treat a narrow range of frequencies very differently than

all other frequencies. These are referred to as resonant circuit. The gain of a highly resonant circuit attains a sharp maximum or minimum as its resonant frequency.

18. When the circuit is said to be in resonance?1. A network is in resonance when the voltage and current at the network input

terminals are in phase.2. If inductive reactance of a network equals capacitive reactance then the

network is said to be resonance

19. What is resonant frequency ?The frequency at which resonance occurs is called resonance frequency.

1f r = ________ 2Л√LC

20. Define bandwidth.The bandwidth (BW) is defined as the frequency difference between upper

cut-off frequency (f2) and lower cut-off frequency (f1)Bandwidth = f2-f1

Where f2 upper cut-off frequencyf1 lower cut-off frequency

21. Define selectivity.Selectivity is defined as the ratio of bandwidth to the resonant frequency

of resonant circuit.Bandwidth

Selectivity = _______________________Resonant frequency

22. Define quality factor.The quality factor is defined as the ratio of maximum energy stored to the

energy dissipated per cycle.Maximum energy stored per cycle

Quality factor (Q) = 2Л * ______________________________Energy dissipated per cycle

23. Define half power frequencies ?The frequencies at which the power is half the maximum power are called

half power frequencies. R

Lower half power frequency, f1 = f r - ____ 4Л L

RUpper half power frequency, f2 = f r + ____

4Л L

24. Write down the formula for inductive reactance and capacitive reactance?

Inductive reactance is given by XL = 2Л fl

1Capacitive reactance is given by XC = _________

2Л fcWhereF supply frequencyL Inductance of the coilC Capacitance of the capacitor.

25. Give the expression for quality factor of series RLC Circuit.Quality factor is Q = 1 / R √ L /C

26. Give the expression for quality factor of parallel RLC Circuit.Quality factor is Q = R √ C / L

UNIT III SEMICONDUCTOR DIODES

1. Give the value of Charge, Mass of an electron.Charge of an electron – 1.6 x 10 -19 coloumbs & Mass of an electron - 9.11 x 10 -31 Kgs

2. Define Potential.A potential of V volts at point B with respect to point A, is defined as the workdone in taking unit positive charge from A to B , against the electric field.

3. Define Current density.It is defined as the current per unit area of the conducting medium. J = I / A

4. Define Electron volts.If an electron falls through a potential of one volt then its energy is 1 electron volt.1 eV = 1.6 x 10 -19 joules

5. What is Electrostatic deflection sensitivity?Electrostatic deflection sensitivity of a pair of deflecting plates of a cathode rayoscilloscope ( CRO) is defined as the amount of deflection of electron spotproduced when a voltage of 1 Volt DC is applied between the corresponding plates.

6. What is the relation for the maximum number of electrons in each shell?Ans: 2n2

7. What are valence electrons?Electron in the outermost shell of an atom is called valence electron.

8. What is forbidden energy gap?The space between the valence and conduction band is said to be forbidden energy gap.

9. What are conductors? Give examples?Conductors are materials in which the valence and conduction band overlap each other so there is a swift movement of electrons which leads to conduction. Ex. Copper, silver.

10. What are insulators? Give examples?Insulators are materials in which the valence and conduction band are far awayfrom each other. So no movement of free electrons and thus no conduction.Ex glass, plastic.

11. Give the energy band structure of Insulator.In Insulators there is a wide forbidden energy gap. So movement of valenceelectron from valence to conduction band is not possible.

12. Give the energy band structure of Semi conductor.In Semiconductors there is a small forbidden energy gap. So movement of valence electron from valence to conduction band is possible if the valence electrons are supplied with some energy.

13. Give the energy band structure of conductor.In conductors there is no forbidden energy gap, valence band and conductionand over lap each other. so there is a heavy movement of valence electrons.

14. what are Semiconductors? Give examples?The materials whose electrical property lies between those of conductors andinsulators are known as Semiconductors. Ex germanium, silicon.

15. What are the types of Semiconductor?1. Intrinsic semiconductor 2. Extrinsic semiconductor.

16. What is Intrinsic Semiconductor?Pure form of semiconductors are said to be intrinsic semiconductor.Ex: germanium, silicon.

17. Define Mass – action law.Under thermal equilibrium the product of free electron concentration (n) and holeconcentration (p) is constant regardless of the individual magnitude.n.p = ni2

18. What is Extrinsic Semiconductor?If certain amount of impurity atom is added to intrinsic semiconductor theresulting semiconductor is Extrinsic or impure Semiconductor.

19. What are the types of Extrinsic Semiconductor?1. P-type Semiconductor 2. N- Type Semiconductor.

20. What is P-type Semiconductor?The Semiconductor which are obtained by introducing pentavalent impurity atom(phosphorous, antimony) are known as P-type Semiconductor.

21. What is N-type Semiconductor?The Semiconductor which is obtained by introducing trivalent impurity atom (gallium, indium) are known as N-type Semiconductor.

22. What is doping?Process of adding impurity to a intrinsic semiconductor atom is doping. The impurity is called dopant.

23. Which charge carriers is majority and minority carrier in N-typeSemiconductor?majority carrier: electron and minority carrier: holes.

24.which charge carriers is majority and minority carrier in P-typeSemiconductor?Majority carrier: holes and minority carrier: electron

25. Why n - type or penta valent impurities are called as Donor impurities?n- type impurities will donate the excess negative charge carriers ( Electrons) and therefore they are reffered to as donor impurities.

26. Why P – type or trivalent impurities are called as acceptor impurity?p- type impurities make available positive carriers because they create holes which can accept electron, so these impurities are said to be as acceptor impurity.

27. Give the relation for concentration of holes in the n- type material?pn = ni2 /NDWherepn - concentration of holes in the n – type semiconductorND - concentration of donor atoms in the n – type semiconductor

28. Give the relation for concentration of electrons in the p - type material?np = ni2 /NAWherenp - concentration of electrons in p- type semiconductorND - concentration of acceptor atoms in the p – type semiconductor

29. Define drift current?When an electric field is applied across the semiconductor, the holes move towards the negative terminal of the battery and electron move towards the positive terminal of the battery. This drift movement of charge carriers will result in a current termed as drift current.

30. Give the expression for drift current density due to electron.Jn = q n μnEWhere,Jn - drift current density due to electronq- Charge of electronμn - Mobility of electronE - applied electric field

31. Give the expression for drift current density due to holes.Jp = q p μp EWhere, Jn - drift current density due to holes q - Charge of holesμp - Mobility of holes E - applied electric field

32. Define the term diffusion current?A concentration gradient exists, if the number of either electrons or holes is greater in one region of a semiconductor as compared to the rest of the region. The holes and electron tend to move from region of higher concentration to the region of lower concentration. This process in called diffusion and the current produced due this movement is diffusion current.

33. Define mean life time of a hole or and electron.The electron hole pair created due to thermal agitation woll disappear as a result of recombination. Thus an average time for which a hole or an electron exist before recombination can be said as the mean life time of a hole or electron.

34. What is the other name of continuity equation? What does it indicate?The other name of continuity equation is equation of conservation of charge.This equation indicates that the rate at which holes are generated thermally just equals the rate at which holes are lost because of recombination under equilibrium conditions.

35. Define Hall effect?If a metal or semiconductor carrying current I is placed in a transverse magnetic field B , an electric field E is induced in the direction perpendicular to both I and B This phenomenon is known as Hall effect.

36. Give some application of Hall Effect.i). Hall Effect can be used to measure the strength of a magnetic field in terms of electrical voltage.ii).It is used to determine whether the semiconductor is p – type or n- type materialiii).It is used to determine the carrier concentrationiv).It is used to determine the mobility.

37. Define the term transition capacitance?When a PN junction is reverse biased, the depletion layer acts like a dielectric material while P and N –type regions on either side which has low resistance act as the plates. In this way a reverse biased PN junction may be regarded as parallel plate capacitor and thus the capacitance across this set up is called as the transition capacitance.CT = A / WWhereCT - transition capacitanceA - Cross section area of the junctionW – Width of the depletion region

38. What is a varactor diode?A diode which is based on the voltage variable capacitance of the reverse biased p-n junction is said to be varactor diode. It has other names such as varicaps, voltacaps.

39. Define the term diffusion capacitance.The diffusion capacitance of a forward biased diode is defined as the rate of change of injected charge with voltage.CD = I / VTWhere, Cd – time constant I – current across the diode vT – threshold voltage

40. what is recovery time? Give its types.When a diode has its state changed from one type of bias to other a transient accompanies the diode response, i.e., the diode reaches steady state only after an interval of time “ tr” called as recovery time. The recovery time can be divided in to two types such as(i) forward recovery time(ii) reverse recovery time

41. What is meant by forward recovery time?The forward recovery time may be defined as the time interval from the instant of 10% diode voltage to the instant this voltage reaches 90% of the final value. It is represented as t f r.

42. What is meant by reverse recovery time?The reverse recovery time can be defined as the time required for injected or the excess minority carrier density reduced to zero , when external voltage is suddenly reversed.

43. Define storage time.The interval time for the stored minority charge to become zero is called storage time. It is represented as t s.

44. Define transition time.The time when the diode has normally recovered and the diode reverse current reaches reverse saturaton current I0 is called as transition time. It is represented as t t

45. What are break down diodes?Diodes which are designed with adequate power dissipation capabilities to operate in the break down region are called as break down or zener diodes.

46. What is break down? What are its types?When the reverse voltage across the pn junction is increased rapidly at a voltage the junction breaks down leading to a current flow across the device. This phenomenon is called as break down and the voltage is break down voltage. The types of break down arei) zener break downii)Avalanche breakdown

47. What is zener breakdown?Zener break down takes place when both sides of the junction are very heavily doped and Consequently the depletion layer is thin and consequently the depletion layer is tin. When a small value of reverse bias voltage is applied , a very strong electric field is set up across the thin depletion layer. This electric field is enough to break the covalent bonds. Now extremely large number of freecharge carriers are produced which constitute the zener current. This process is known as zener break down.

48. What is avalanche break down?When bias is applied , thermally generated carriers which are already present in the diode acquire sufficient energy from the applied potential to produce new carriers by removing valence electron from their bonds. These newly generated additional carriers acquire more energy from the potential and they strike the lattice and create more number of free electrons and holes. This process goeson as long as bias is increased and the number of free carriers get multiplied. This process is termed as avalanche multiplication. Thus the break down which occur in the junction resulting in heavy flow of current is termed as avalanche break down.

49. How does the avalanche breakdown voltage vary with temperature?In lightly doped diode an increase in temperature increases the probability of collision of electrons and thus increases the depletion width. Thus the electrons and holes needs a high voltage to cross the junction. Thus the avalanche voltage is increased with increased temperature.

50. How does the zener breakdown voltage vary with temperature?In heavily doped diodes, an increase in temperature increases the energies of valence electrons, and hence makes it easier for these electrons to escape from covalent bonds. Thus less voltage is sufficient to knock or pull these electrons from their position in the crystal and convert them in to conduction electrons. Thus zener break down voltage decreases with temperature.

UNIT IV

TRANSISTORS1. What is a transistor (BJT)?Transistor is a three terminal device whose output current, voltage and /or poweris controlled by input current.

2. What are the terminals present in a transistor?Three terminals: emitter, base, collector.

3. What is FET?FET is abbreviated for field effect transistor. It is a three terminal device with its output characteristics controlled by input voltage.

4. Why FET is called voltage controlled device?The output characteristics of FET is controlled by its input voltage thus it is voltage controlled.

5. What are the two main types of FET?1. JFET 2. MOSFET.

6. What are the terminals available in FET?1). Drain, 2).Source and 3). Gate

7. What is JFET?JFET- Junction Field Effect Transistor.

8. What are the types of JFET?N- Channel JFET and P- Channel JFET

9. What are the two important characteristics of JFET?1. Drain characteristics 2. Transfer characteristics.

10. What is transconductance in JFET?It is the ratio of small change in drain current to the corresponding change in drain to source voltage.

11. What is amplification factor in JFET?It is the ratio of small change in drain to source voltage to the corresponding change in Gate to source voltage.

12. Why do we choose q point at the center of the loadline?The operating point of a transistor is kept fixed usually at the center of the active region in order that the input signal is well amplified. If the point is fixed in the saturation region or the cut off region the positive and negative half cycle gets clipped off respectively.

13. List out the different types of biasing. ._Voltage divider bias, Base bias, Emitter feed back bias, Collector feedback bias, Emitter bias.

14. What do you meant by thermal runway?Due to the self heating at the collector junction, the collector current rises. This causes damage to the device. This phenomenon is called thermal runway.

15. Why is the transistor called a current controlled device?The output characteristics of the transistor depend on the input current. So thtransistor is called a current controlled device.

16. Define current amplification factor?It is defined as the ratio of change in output current to the change in input current at constant.

17. What are the requirements for biasing circuits?• The q point must be taken at the Centre of the active region of the output characteristics.• Stabilize the collector current against the temperature variations.• Make the q point independent of the transistor parameters.• When the transistor is replaced, it must be of same type.

18. When does a transistor act as a switch?The transistor acts as a switch when it is operated at either cutoff region or saturation region

19. What is biasing?To use the transistor in any application it is necessary to provide sufficient voltage and current to operate the transistor. This is called biasing.

20. What is stability factor?Stability factor is defined as the rate of change of collector current with respect to the rate of change of reverse saturation current.

21. Explain about the various regions in a transistor?The three regions are active region saturation region cutoff region.

22. Explain about the characteristics of a transistor?Input characteristics: it is drawn between input voltage & input current while keeping output voltage as constant. Output characteristics: It is drawn between the output voltage &output current while keeping input current as constant.

16 marks-Hints

1. Explain the construction, operation, volt ampere characteristics, and applicationof SCR, also explain its two transistor model.Maximum mark for this question: 16 marksConstruction (2marks)Equivalent circuit and two transistor model (2marks)Operation (4marks)Volt ampere characteristics (4marks)Application (2marks)

2 . Explain the construction, operation, equivalent circuit, volt amperecharacteristics, and application of UJT.Maximum mark for this question: 16 marksConstruction (4marks)Equivalent circuit (2marks)Operation (4marks)Volt ampere characteristics (4marks)Application (2marks)

3. Explain the construction, operation, equivalent circuit, volt amperecharacteristics, and application of DIAC.Maximum mark for this question: 16 marksConstruction (4marks)Equivalent circuit (2marks)Operation (4marks)Volt ampere characteristics (4marks)Application (2marks)

4. Explain the construction, operation, equivalent circuit, volt amperecharacteristics, and application of TRIACMaximum mark for this question: 16 marksConstruction (4marks)Equivalent circuit (2marks)Operation (4marks)Volt ampere characteristics (4marks)Application (2marks)

UNIT V

SPECIAL SEMICONDUCTOR DEVICES

1. What is a TRIAC?TRIAC is a three terminal bidirectional semiconductor switching device. It can conduct in both the directions for any desired period. In operation it is equivalent to two SCR’s connected in antiparallel.

2. Give the application of TRIAC.1. Heater control2. Motor speed control3. Phase control4. Static switches

3. What are the different operating modes of TRIAC?1. Keeping MT2 and G positive2. Keeping MT2 and G negative.3. Keeping MT2 positive and G negative.4. Keeping MT2 negative and G positive.

4. What are the regions in the VI characteristics of UJT?1. Cut-off region2. Negative resistance region.3. Saturation region

5. What is meant by negative resistance region of UJT?In a UJT when the emitter voltage reaches the peak point voltage, emitter current starts flowing. After the peak point any effort to increase in emitter voltage further leads to sudden increase in the emitter current with corresponding decrease in emitter voltage, exhibiting negative resistance. This takes place until the valley point is reached. This region between the peak point and valley point is called negative resistance region.

6. Mention the applications of UJT.1. It is used in timing circuits2. It is used in switching circuits3. It is used in phase control circuits4. It can be used as trigger device for SCR and triac.5. It is used in saw tooth generator.6. It is used for pulse generation.

7. What is a DIAC?DIAC is a two terminal bidirectional semiconductor switching device. . It can conduct in either direction depending upon the polarity of the voltage applied across its main terminals. In operation DIAC is equivalent to two 4 layer diodes connected in antiparallel.

8. Give some applications of DIAC.1. To trigger TRIAC2. Motor speed control3. Heat control4. Light dimmer circuits

9. What is a SCR?A silicon controller rectifier (SCR) is a three terminal, three junction semiconductor device that acts as a true electronic switch. It is a unidirectional device. It converts alternating current into direct current and controls the amount of power fed to the load.

10. Define break over voltage of SCR.Break over voltage is defined as the minimum forward voltage with gate open at which the SCR starts conducting heavily.

11.Why SCR cannot be used as a bidirectional switch.SCR can do conduction only when anode is positive with respect to cathode with proper gate current. Therefore, SCR operates only in one direction and cannot be used as bidirectional switch.

12. How turning on of SCR is done?1. By increasing the voltage across SCR above forward break over voltage.2. By applying a small positive voltage at gate.3. By rapidly increasing the anode to cathode voltage.4. By irradiating SCR with light.

13. How turning off of SCR is done?1. By reversing the polarity of anode to cathode voltage.2. By reducing the current through the SCR below holding current.3.By interrupting anode current by means of momentarily series or parallel switching

14. Define holding current in a SCR.Holding current is defined as the minimum value of anode current to keep the SCR ON.

15. List the advantages of SCR.1. SCR can handle and control large currents.2. Its switching speed is very high3. It has no moving parts, therefore it gives noiseless operation.4. Its operating efficiency is high.

16. List the application of SCR.1. It can be used as a speed controller in DC and AC motors.2. It can be used as an inverter.3. It can be used as a converter4. It is used in battery chargers.5. It is used for phase control and heater control.

6. It is used in light dimming control circuits.

17. What is meant by latching.The ability of SCR to remain conducting even when the gate signal is removed is called as latching.

18. Define forward current rating of a SCR.Forward current rating of a SCR is the maximum anode current that it can handle without destruction.

19. List the important ratings of SCR.1. Forward break over voltage2. Holding current3. Gate trigger current4. Average forward current5. Reverse break down voltage.

20. Compare SCR with TRIAC. SCR TRIAC1. unidirectional current 1. bidirectional current2. triggered by positive pulse at gate 2. triggered by pulse of positive or negative at gate3. fast turn off time 3,. Longer turn off time4. large current ratings 4. lower current ratings

21. Differentiate BJT and UJT.BJT UJT

1. It has two PN junctions 1. It has only one PN junctions2. three terminals present 2. three terminals present are emitter, are emitter, base,collector base1,base23. basically a amplifying device 3. basically a switching device

22. What is Shockley diode (PNPN diode)?Shockley diode is a four layered PNPN silicon diode. It is a low- current SCR without a gate. This device is switched ON when the anode to cathode voltage is increased to forward switching voltageVS which is equivalent to SCR forward break over voltage.

23. What is a thyristor?Thyristor is a semiconductor device having three or more junctions .Such a device acts as a switch without any bias and can be fabricated to have voltage ratings of severalhundred volts and current ratings from a few amperes to almost thousand amperes.

24. What are the types of thyristors?1. Unidirectional thyristors2. Bidirectional thyristors3. Low-power thyristors

25. Give the various triggering devices for thyristors.1. SCR2. UJT3. DIAC4. TRIAC

26. what is backward diode?The backward diode is a diode in which the doping level is moderate. The forward current in this case is very small, very much similar to that of the reverse current in the conventional diode.

27. what is a photo diode?The photo diode is a diode in which the current sensitivity to radiation can be made much larger by the use of the reverse biased PN junction. Thus this diode conducts heavily in the reverse bias when there is some radiaton allowed to fall on the PN junction.

28. What is a LED?A PN junction diode which emits light when forward biased is known as Light emitting diode (LED).

29. What is a tunnel diode?The tunnel diode is a pn junction diode in which the impurity concentration is greatly increased about 1000 times higher than a conventional PN junction diode thus yielding a very thin depletion layer. This diode utilizes a phenomenon called tunneling and hence the diode is referred as tunnel diode.

30. What is tunneling phenomenon?The phenomenon of penetration of the charge carriers directly though the potential barrier instead of climbing over it is called as tunneling.