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Transistors

Unit-4

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WHAT IS A TRANSISTOR?

A Transistor is a electronic device composed

of layers of a semiconductor material which

regulates current or voltage flow and acts asa switch or gate for electronic circuit.

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APPLICATIONS

AMPLIFICATION

SWITCHING

OSCILATING CIRCUIT

SENSORS

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Bipolar Junction Transistor

First created in 1948 by Bell

Telephone

Similar to the Mosfet

Reliable under severeconditions dominant in

automobiles

NPN and PNP

Digital logic circuits-Boolean

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Introduction

The basic of electronic system nowadays is

semiconductor device.

The famous and common use of this device isBJTs

(Bipolar Junction Transistors).

It can be use as amplifier and logic switches.

BJT consists of three terminal:

collector : C

base : B

emitter : E

Two types of BJT : pnp and npn

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Transistor Construction

3 layer semiconductor device consisting:

2 n- and 1 p-type layers of material npn transistor

2 p- and 1 n-type layers of material pnp transistor

The term bipolar reflects the fact that holes andelectrons participate in the injection process into the

oppositely polarized material A single pn junction has two different types of bias:

forward bias

reverse bias

Thus, a two-pn-junction device has four types of bias.

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Position of the terminals and symbolof BJT.

Base is located at the middleand more thin from the levelofcollectorandemitter

The emitter and collectorterminals are made of thesame type of semiconductormaterial, while the base of theother type of material

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Transistor currents

-The arrow is always drawn

on the emitter

-The arrow always point

toward the n-type

-The arrow indicates the

direction of the emitter

current:

pnp:E B

npn: B

E

IC=the collector current

IB= the base currentIE= the emitter current

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By imaging the analogy of diode, transistor can be

construct like two diodes that connetecd together. It can be conclude that the work of transistor is base on

work of diode.

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Transistor Operation

The basic operation will be described using the pnptransistor. The operation of the pnp transistor isexactly the same if the roles played by the electronand hole are interchanged.

One p-n junction of a transistor is reverse-biased,whereas the other is forward-biased.

Forward-biased junctionof a pnp transistor

Reverse-biased junctionof a pnp transistor

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Both biasing potentials have been applied to a pnptransistor and resulting majority and minority carrierflows indicated.

Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.

A very small number of carriers (+) will through n-typematerial to the base terminal. Resulting IB is typically inorder of microamperes.

The large number of majority carriers will diffuse acrossthe reverse-biased junction into the p-type material

connected to the collector terminal.

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Majority carriers can cross the reverse-biased

junction because the injected majority carriers willappear as minority carriers in the n-type material.

Applying KCL to the transistor :

IE= IC+ IB

The comprises of two components the majority

and minority carriers

IC= ICmajority+ ICOminority

ICO ICcurrent with emitter terminal open and is

called leakage current.

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Common-Emitter Configuration

It is called common-emitter configuration since :

- emitter is common or reference to both input and

output terminals.

- emitter is usually the terminal closest to or at

groundpotential.

Almost amplifier design is using connection of CE due

to the high gain for current and voltage.

Two set of characteristics are necessary to describethe behavior for CE ;input (base terminal) and output

(collector terminal) parameters.

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Proper Biasing common-emitter configuration in active region

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Input characteristics for acommon-emitter NPN transistor

IB is microamperes compared

to miliamperes of IC.

IB will flow when VBE > 0.7V

for silicon and 0.3V forgermanium

Before this value IB is verysmall and no IB.

Base-emitter junction isforward bias

Increasing VCE will reduce IB

for different values.

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Output characteristics for acommon-emitter npntransistor

For small VCE (VCE < VCESAT, IC increase linearly withincreasing of VCE

VCE > VCESAT IC not totally depends on VCE constant IC

IB(uA) is very small compare to IC (mA). Small increase inIB cause big increase in IC

IB=0 A ICEO occur.

Noticing the value when IC=0A. There is still some value of

current flows.

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Beta () or amplification factor The ratio of dc collector current (IC) to the dc base current

(IB) is dc beta (dc ) which is dc current gain where ICand IB are determined at a particular operating point, Q-

point (quiescent point).

Its define by the following equation:

30 < dc < 300 2N3904

On data sheet, dc=hFE with h is derived from ac hybridequivalent cct. FE are derived from forward-current

amplification and common-emitter configuration

respectively.

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For ac conditions an ac beta has been defined as the

changes of collector current (IC) compared to the

changes of base current (IB) where IC and IB aredetermined at operating point.

On data sheet, ac=hfe

It can defined by the following equation:

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Relationship analysis between and

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Transistor Terminal Identification

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Disadvantage of BJT

Downfalls of BJT- Large base current, ib,

is needed to turn transistor on.

Electrons and holes contribute toconduction which slows down the

switching speed.

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Construction of n-channel

enhancement MOSFET

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Basic operation

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Transfer characteristics and Drain

chacracteristics

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CMOS

CMOS-Complementary MOS

P-channel and n-channel is constructed on

the same substrate

Has extensive application in computer logic

design

It is effectively used as an Inverter.

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ADVANTAGES

Fast switching speed

Low operating power levels

High input impedence

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CMOS -INVERTER