INTRODUCTION TO ELECTRONICS

COURSE STRUCTURE

1. Characteristics of Semi-conductors, conductors and Insulators

2. PN Junction and Diode Biasing

3. Junction Barrier, Junction Breakdown

4. Types of Diodes

5. Junction Transistors (BJJ, FET)

6. Electronic Amplifiers and Coupling of Amplifiers

7. Rectifiers

8. Power Supplies and Filters

9. Multipliers and Voltage Regulators

10. Revision

CHARACTERISTICS OF SEMI-CONDUCTORS

Semi conductors are materials whose electrical conductivity lies between

that of very good conductors and very good insulators. Examples are Silicon

[the most commonly used semiconductor material] (si) and Germanium [and

also Carbon (not suitable for electronic activities)] (ge).

Conductors:

They are materials that allow the flow of electric current through them.

Examples are Copper, Aluminium, etc.

Insulators:

These are materials that do not allow the flow of electric current through

them. Examples are wood, paper, plastic, etc.

ENERGY BAND DIAGRAMS

CONDUCTORS:

Conduction Band

Overlapping forbidden gap

Valence Band

Here, both the conduction band and valence band overlap hence conductivity becomes easy.

SEMI-CONDUCTORS

Conduction Band

Forbidden Gap(narrow)

Valence Band

INSULATORS

Conduction Band

Wide Forbidden Gap

Valence Band

INTRINSIC AND EXTRINSIC SEMIDCONDUCTORS

Intrinsic (pure) semi-conductors are those that have no impurities in them. Examples are germanium and Silicon.

Extrinsic semi-conductors are those obtained after doping. Doping is a process by which an impurity or impurities are added to a pure semi-conductor to improve its conductivity. Examples and types include P-Type and N-Type semi-conductors.

P-Type Semi-conductor – Formation

When a small amount of trivalent (having a valence of three) impurity or element such as boron or aluminum is added to the pure semi-conductor such as Germanium or Sillicon, a P-Type semi-conductor is formed.

Note: Germanium atom has 4 valence electrons and Boron atom has three valence electrons.

Therefore trivalent impurity such as Boron, when added to the intrinsic semi-conductor, such as germanium introduces a large number of holes in the valence band. This positively charged holes increase the conductivity of P-Type semi-conductor therefore for the formation of the P-Type semi-conductor, holes are majority carriers and electrons are minority carriers

N-Type Semi-conductor – Formation

When a small amount of pentavalent element (elements of five electrons in their outermost shell) or impurity is added to a pure semi-conductor such as phosphorus, then the N-Type semi-conductor is formed.

NOTE: Germanium atom has four valence electrons and phosphorus or arsenic has five valence electrons.

Therefore pentavalent impurity such as phosphorus, when added to a pure semi-conductor such as germanium, introduces a large number of electrons in the valence band. These negatively charged electrons increase the conductivity of N-Type semi-conductor. Hence for the formation of the N-Type semi-conductor, electrons are majority carriers and holes are minority carriers.

THE PN JUNCTION

The PN Junction

When P-Type material is welded or joined to an N-Type material, a junction is created known as the PN Junction.

Depletion layer

When the PN Junction is formed, the mobile charges in the vicinity of the junction are strongly attracted to their opposite and drifts towards the

P N

- + - +

P - + N

junction. As the charges accumulate, the action increases. Some electrons move across the junction and fill some of the holes near the junction in the P-Type material. Also in the N-Type material, the electrons become depleted near the junction. This region near the junction where holes and electrons are depleted is called the depletion region or layer.

BARRIER POTENTIAL (Barrier Voltage)

The opposite charges that build up on each side of the junction produces or creates a voltage which is referred to as Barrier Voltage (potential).

Diode:

A diode is a device that allows current to flow through it in only one direction. Examples are PN Junction diode, Zener diode and light emitting diode (LED)

Assignment: State any two types of diodes and explain the principles of one.

METHODS OF FABRICATION OF PN JUNCTION DIODE

1. Growth Junction Method

2. Alloyed Junction Method

3. Diffused Junction Method

BIAS VOLTAGE

When a voltage is applied to a diode, it is referred to as bias voltage

Forward Bias Reverse Bias

P N P N

Here, the P-Type material is connected to the positive terminal of the battery and the N-Type material is connected o the negative terminal of the battery

Here, the P-Type material is connected to the negative terminal of the battery and the N-Type material is connected o the negative terminal of the battery

CHARACTERISITICS CURVE OF THE PN JUNCTION

I (MA)

Forward Bias

-V (v) V (v)

Reverse Bias

-I (MA)

CHARACTERISITCS OF GERMANIUM AND SILLICON

I (MA)

Ge

si

-V(v) V (v)

-I (MA)

Assignment: Why is Silicon preferred to Germanium? How does doping support current flow in a semi-conductor material? Describe the 3 methods of diode fabrication.

Ohms Law: It states that the Potential difference across a conductor is directly proportional to the current flowing through the conductor provided the temperature and other physical conditions remain constant. V=IR

Zena Diode:

Anode (+) cathode (-)

A zener diode is a sharp breakdown of voltage. The applied reverse voltage at which the breakdown occurs is called breakdown voltage or peak reverse voltage.

Properties of Zener Diode

- It is a highly doped crystal diode with a sharp breakdown voltage

- It is always connected in a reverse bias

- It behaves as an ordinary diode when connected in a forward biased

Note: A breakdown voltage of a zener diode is determined by the resistivity of the diode

CHARACTERISTICS CURVE OF A ZENER DIODE

I (mA)

Forward Bias

-V (v) V (v)

Reverse Bias

-I (mA)

The ability of a zener diode to dissipate power decreases as the temperature increases therefore power dissipation ratings are given for specific temperatures. The primary function of a zener diode is to stabilize voltage or current. Zener diodes are used to stabilize or regulate current.

Bipolar Junction Transistor:

1. PNP

2. NPN

Construction of PNP

Emitter Collector

Base

Here an N-Type material is sandwiched between two p-type materials of a semi-conductor material.

NOTE: Emitter, Collector and Base are the three terminals.

Emitter Collector

Base

Here a P-Type material is sandwiched between two n-type materials of a semi-conductor material

P N P

N P N

PRACTICE QUESTIONS

Explain the function of each of the following

- Emitter

- Collector

- Base

TRANSISTOR BIASING

To bias a transistor, the emitter-based junction is forward biased and the collector based junction is reverse biased. Due to forward bias on the emitter-based junction, an emitter current flows through the base into the collector. Though the collector-based junction is reverse biased, almost the entire current flows through the collector circuit because of the large electric potential on the collector side.

Types of Configuration

1. Common Emitter (CE) configuration

2. Common Base (CB) Configuration

3. Common Collector (CC) Configuration

CE CONFIGURATION

C

B V EC

VBF E

CB configuration

Note the following

1. BJT –

NEGATIVE TEMPERATURE CO-EFFICIENCT OF A MATERIAL

This occurs when the conductivity of a material decreases as its temperature increases

CONSTRUCTION OF P-CHANNEL JFET

Source Terminal Gate Terminal Drain Terminal

P- Channel

N-Substrate

Write on the operation: How conductivity occurs in the P-Channel JFET

MOSFET

This is also known as Depletion insulated Gate Field Effect Transistor. This device do not use a PN Junction, instead they use a metal gate which is electrically isolated from the semi-conductor channel by a thin layer of oxide. There are two types of MOSFET; N-Channel MOSFET and P-Channel MOSFET. The N-Type unit with N-Channel are called depletion mode devices because they conduct when zero bias is applied to the gate. In the depletion mode,

N

the electrons are conducting until they are depleted by the gate bias voltage.

CONSTRUCTION OF N-CHANNEL MOSFET

Metal Gate

Source Insulating Layer Drain

P substrate

The N-Channel depletion MOSFET is formed by implanting an N-Channel in a P Substrate. A thin insulating layer of Silicon Dioxide is then deposited on the channel, leaving the ends of the channel exposed to be attached to wires and act as the source and the drain. A thin metallic is attached to the insulating layer over the N-Channel. The metallic layer serves as the gate. An additional LED is attached to the substrate. The metal gate is insulated from the semi-conductor channel so that the gate and the channel do not form a P-N Junction. The metal gate is used to control the conductivity of the channel as in the JFET

Insulating layer

Source Metal Gate Drain

P

P

N-Substrate

Question:

- Describe how a JFET differs in construction from a Bipolar Transistor

- Define the following with reference to JFET; Depletion region, Pinch-off voltage, Source, Drain

- How does a MOSFET differ in construction from a JFET?

- Describe how a MOSFET conducts a current

ELECTRONIC AMPLIFIERS

Electronic devices used to

Amplifier configurations:

Note: In order for a transistor to provide amplification, it must be able to accept an input signal and produce an output signal that is greater than the input. The configurations are as follows:

- Common Emitter (CE) Configuration

- Common Base (CB) Configuration

- Common Collector (CC) Configuration

CHARACTERSTICS

Circuit Type Input Resistanc

e

Output Resistance

Voltage Gain

Current Gain

Power Gain

Common Low High High Less Medium

Base Than 1

Common Emitter

Medium Medium Medium Medium High

Common Collector

High Low Less Than 1

Medium Medium

Note: Voltage Gain = Output Voltage Input Voltage

Current Gain = Output Current Input Current

Power Gain = Voltage Gain x Current Gain

The Input – Output waveforms of the three configurations are given below:

Amplifier Type Input Waveform Output WaveformCommon Base

Common Emitter

Common Collector

AMPLIFIER COUPLING

To obtain greater amplification, transistor amplifiers, may be connected together. However, to prevent one amplifier’s biased voltage from affecting the operation of the second amplifier, a coupling technique must be used.

Note: The coupling method used must not disrupt the operation of the either circuit. The coupling methods are as follows:

- Resistance – capacitance coupling- Impedance coupling- Transformer coupling- Direct Coupling

Resistance-Capacitor coupling is primarily used in audio amplifiers. Here, resistors and capacitors are used for the coupling. The coupling capacitor is generally and electrolytic type.

Impedance coupling works just like resistance-capacitance coupling. The advantage is that the inductor has a very low DC resistance across its windings. The disadvantage of impedance coupling is that the inductance reactance increases with the frequency. The voltage gain varies with the frequency. This type of coupling is ideal for single frequency amplification when a very narrow band of frequencies must be amplified.In a transformer coupled circuit, the two amplifier stages are coupled through the transformer. Note: The transformer can effectively match a high impedance source to a low impedance load. The disadvantage of transformer coupling is that transformers are large and expensive. Transformer coupling are used only for a narrow band of frequencies. Direct coupling techniques are used when a very low frequency or DC signal must be amplified. They are used for low frequency devices or amplifiers.

POWER SUPPLIES

Power Supplies are used to supply voltage to a variety of circuits. Note: The primary function of a power supply is to convert an alternating current (AC) to Direct Current (DC). The power Supply may increase or decrease the incoming AC voltage by means of a transformer.

TRANSFORMERS

Transformers are used in power supplies to isolate the power supply from the AC voltage source. Note: They are also used to step up voltages if higher voltages are required or to step down voltages if lower voltages are required. If transformers are used in power supplies, the AC power source is connected only to the primary of the transformer. This isolates the electrical circuit from the power source.

RECTIFIERS

Note: The rectifier circuit is the heart of the power supply. Its function is to convert the incoming AC voltage to a DC voltage.

Types of Rectifier Circuits

Half wave rectifierFull wave rectifierBridge rectifier