n pn pnp transistor lesson 01 a

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2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 1

Transistor and their

Characteristics

Lesson-1 Junction Transistor


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1.1. Junction TransistorJunction Transistor

DefinitionDefinition

The transferred-resistance or transistor

is a multi-junction device that is capableof

Current gain

Voltage gain Signal-power gain


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Bipolar junction transistorBipolar junction transistor

Invented in 1948 by Bardeen, Brattain andShockley

Contains three adjoining, alternately doped

semiconductor regions: Emitter (E), Base (B), andCollector (C)

The middle region, base, is very thin compared

to the diffusion length of minority carriers

Two kinds: npn and pnp


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2008Junction Transistor Unit 4 Lesson 1-

" , Raj Kamal,4

Bipolar junction transistor BJT

Definition

The Bipolar junction transistor is an active

device that works as a voltage-controlledcurrent source and whose basic action is

control of current at one terminal by

controlling voltage applied at other two

terminals.


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Bipolar Junction Transistor Representation

NPN PNP


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2.2. Transistor CurrentTransistor Current

ComponentsComponents


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BJT circuit Current ComponentsBJT circuit Current Components

As shown, the currents are positive quantities when the

transistor is operated in forward active mode.

IE =IB +IC and VEB + VBC + VCE = 0 VCE = VEC


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Meaning of Forward biasingMeaning of Forward biasing

When the p side is applied +ve and n side

negative in a junction and applied voltage is

grater than a threshold 0.65 V for Silicon

(Si)


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Meaning of Reverse biasingMeaning of Reverse biasing

When the p side is applied -ve and n side

+ve in a junction and applied voltage is

between 0 to a breakdown voltage


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3.3. Transistor as AmplifierTransistor as Amplifier


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BJT CommonBJT Common

BaseBase

configurationconfiguration


EmitterEmitter



CollectorCollector


PNPPNP PNP

IBIE IC

Three configurations


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" , Raj Kamal,

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Six Problems

Draw circuits of CB, CE, CC

configurations for npn and pnp transistors Show current directions


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" , Raj Kamal,

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npn transistor ina simple circuit, known as

common-emitteramplifier


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Characteristics ofCharacteristics of Transistor Amplifier in threeTransistor Amplifier in three

regions of operationsregions of operations


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Input or driving PointInput or driving Point

Characteristics of CommonCharacteristics of Common

Base transistor AmplifierBase transistor Amplifier

Input or driving Point

Characteristics of Common

Emitter transistorAmplifier

Active region

Saturation region Saturation region

Active region


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Output CollectorOutput Collector


Base transistor AmplifierBase transistor Amplifier

Output CollectorOutput Collector


Emitter transistorEmitter transistor

AmplifierAmplifier

Saturation region Cut-off region Cut-off regionSaturation region

Active region Active region


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First Approximation RelationshipFirst Approximation Relationship

betweenbetween IICC andand IIEE IC IE


BaseBase



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" , Raj Kamal,

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The collector current is being controlled by the B-E voltage, orthe current in the one part of the device is being controlled bythe voltage in another part - transistor action

Since the B-E junction is forward biased, holes from the baseare injected into the emitter. However, these injected holesdo not contribute to the collector current and are thereforenot part of the transistor action

To design a useful device, we need mathematical expressionsfor the minority carrier concentrations shown in the figureabove.

There are three modes of operation we must consider

Forward-active (B-E FB, B-C RB) Cut-off (B-E RB, B-C RB) Saturation (B-E FB, B-C FB)


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BJT biasing modesBJT biasing modes


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In the cut-off region, the base emitter and

collector base junctions of transistor are

reverse biasedIC IE 0

CutCut--off defined as region of characteristics Ioff defined as region of characteristics ICC = 0= 0


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Saturation- region defined as region of

characteristics left of VCB = 0 and IC increase

exponentially with VCB increases toward 0

In the saturation region region, the base

emitter and collector base junctions of

transistor are forward biased


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Active - region defined as region of

characteristics right of VCB

= 0 and IC

first

increases exponentially with VCB increases

toward 0.7 V in Si transistor and then

becomes constant

In the active region region, the base emitter andcollector base junctions of transistor are forward

and reversed biased respectively IC IE 0and is in mA IC


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Operation in theforward-active

mode

B-E junction is forward biased so

electrons

can be injected from the emitter to the

base, B-C

junction is reverse biased.


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4.4. TransistorTransistor ConstructionConstruction


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" , Raj Kamal,

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BJT structure - three regions, two p-n junctions, threeterminals (emitter, base, collector)

ThinThin


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ConstructionConstruction

Devices can be p-n-p, or n-p-n structures

Width of the base is small compared to the

minority carrier length and is about 1/150 of

total width, number of free carriers aresmall as doping level is 1/10 th or less

compared to collector)

Emitter is normally heavily doped, thecollector has light doping


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SummarySummary

We learnt

(i) pnp transistor

(ii) npn transistor (iii) Current Components

(iv) Three configurations-

Common base, common emitter

and Common collector


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We learnt

(v) Three regions- cut-off,

saturation and active

(vi) Transistor construction


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End of Lesson 1End of Lesson 1

n pn pnp transistor lesson 01 a

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