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Bipolar Junction Transistor

The Transistor The Transistor was invented by a team of

three man at Bell Laboratories in 1947

The transistor is used in two broad area as

a linear amplifier to boost or amplify an

electrical signal & as an electronic switch

All of the complex electronic devices &

systems today are an outgrowth of earlydevelopments in semiconductor transistors

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Transistor Construction

The Bipolar Junction Transistor (BJT) isconstructed with three doped semiconductorregions separated by two pn junctions

The three regions are called Emitter

Base

Collector

There are two types of BJT

npn - Two n-regions separated by a p-region pnp - Two p-regions separated by a n-region

Transistor Construction The pn junction joining the base region &

the emitter region is called the base-emitter junction

The pn junction joining the base region &the collector region is called the base-collector junction

The base region is lightly doped & verythin compared to the heavily doped emitter& the moderately doped collector regions

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Basic Transistor Operation In order for the transistor to operate properly as

an amplifier, the two pn junctions must becorrectly biased with external dc voltages

The operation of the pnp is the same as for thenpn except that the roles of the electrons &holes, the bias voltage polarities, & the currentdirections are all reversed

Notice that in both cases the base-emitter (BE)junction is forward-biased & the base-collector(BC) junction is reverse-biased

This is called forward-reverse bias

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Forward Reverse Bias

Basic Transistor Operation The forward bias from the base to emitter narrows the

BE depletion region, & the reverse bias from base tocollector widens the BC depletion region

The heavily doped n-type emitter is teeming withconduction band free electrons that easily diffusethrough the forward-biased BE junction into the p-region,

just as in a forward-biased diode

The base region is lightly doped & very thin so that it hasa very limited number of holes

Thus, only a small percentage of all the electrons flowing

through the BE junction can combine with the availableholes in the base, forming the small base electroncurrent

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Basic Transistor Operation

Most of the electrons flowing from the emitter

into the thin, lightly doped base region do not

recombine but diffuse into the BC depletion

region

Once in this region the electrons as being pulled

across the reverse-biased BC junction by the

attraction of the collector positive supply voltage

The electrons now move through the collectorregion forming the collector electron current

Basic Transistor Operation

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Transistor Currents

The emitter current is the sum of the

collector current & base current

Notice that the arrow on the emitter of the

transistor symbols points in the direction of

conventional flow current

Transistor DC Bias

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DC Beta & DC Alpha

The ratio of the dc collector current to the

dc base current is the dc beta which is the

dc current gain of a transistor

The ratio of the dc collector current to the

dc emitter current is the dc alpha

DC Beta & DC Alpha DC is usually designated as an equivalent

hybrid (h) parameter, hFE on transistor

data sheets

Relationship of DC

& DC

, this equation

shows that the closerDC is to 1, the

higher of value of DC

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Current & Voltage Analysis

Consider the basic transistor bias circuit

configuration. Three transistor dc currents &

three dc voltages can be identified

IB dc base current

IE dc emitter current

IC dc collector current

VBE dc voltage at the base with respect to emitter

V

CB dc voltage at collector with respect to base VCE dc voltage at collector with respect to emitter

Current & Voltage Analysis

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Collector Characteristic Curves

Assume that VBB is set to produce a certain value ofIB &VCC is zero

As VCC is increased, VCE increases gradually as thecollector current increases

IC increases as VCC is increased because VCE remainsless than 0.7V due to the forward-biased base-collector

junction

Ideally, when VCE exceeds 0.7V, the base-collectorjunction becomes reverse-biased & the transistor goesinto the active or linear region of its operation

When VCE reaches a sufficiently high voltage, thereverse-biased base-collector junction goes intobreakdown

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Collector Characteristic Curves

A family of collector characteristic curves

is produced when IC versus VCE is plotted

for several values ofIB

When IB = 0, the transistor is in the cutoff

region although there is a very small

collector leakage current

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Cutoff Condition

When IB = 0, the transistor is in the cutoff region

of its operation

Under this condition, there is a very small

amount of collector leakage current, ICEO due

mainly to thermally produced carriers

ICEO is extremely small, it will usually be

neglected in circuit analysis so that VCE=VCC

In cutoff, both the base-emitter & the base-collector junctions are reverse-biased

Cutoff Condition

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Saturation Condition

When the base-emitter junction becomes

forward-biased & the base current is increased,

the collector current also increases & VCEdecreases as a result of more drop across the

collector resistor

When VCE reaches its saturation value, VCE(Sat),

the base-collector junction becomes forward-

biased & IC can increase no further even with acontinued increase in IB

Saturation Condition

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DC Load Line

Cutoff & Saturation can be illustrated in

relation to the collector characteristic

curves by the use of a load line

In between cutoff & saturation along the

load line is the active region of the

transistors operation

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More About DC

The DC orhFE is a very important bipolar

junction transistor parameter that we need

to examine further

DC is not truly constant but varies with

both collector current & with temperature

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Maximum Transistor Rating

The transistor, like any other electronic

device, has limitations on its operation

These limitations are stated in the form of

maximum ratings

The product ofVCE & IC must not exceed

the maximum power dissipation

Both VCE & IC cannot be maximum at thesame time

Maximum Transistor Rating

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Derating Factor

PD(max) in data sheet is usually specified at

25C

For higher temperatures, PD(max) is less

For example, a derating factor of 2mW/C

indicates that the maximum power

dissipation is reduced 2mW for each

centigrade degree increase in temperature

The Transistor as an Amplifier Amplification is the process of linearly increasing

the amplitude of an electrical signal & is one of

the major properties of a transistor

A transistor amplifies current because the

collector current is equal to the base current

multiplied by the current gain

The base current in a transistor is very small

compared to the collector & emitter currents

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DC & AC Quantities

DC quantities always carry an uppercase

roman non-italic subscript

Example IB, VCE

AC & all time varying quantities always

carry a lowercase italic subscript

Example Ib, Vin

Basic Transistor Amplifier

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The Transistor as a Switch

The second major application area is

switching applications

When used as an electronic switch, a

transistor is normally operated alternately

in cutoff & saturation

Digital circuits make use of the switching

characteristics of transistors

The Transistor as a Switch The transistor is in the cutoff region because the

base-emitter junction is not forward-biased.There is ideally an open circuit between collector& emitter

The transistor is in the saturation regionbecause the base-emitter junction & the base-collector junction are forward-biased & the basecurrent is made large enough to cause the

collector current to reach its saturation value.There is ideally, a short circuit between collector& emitter

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The Transistor as a Switch

A simple Application of a Transistor

Switch

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Why Transistor

The basic amplifying or switching action

was produced by transferring a current

from a low to a high resistance circuit or

vice-versa

The combination of the two terms transfer

+ resistorresults in the label transistor

Transistor Packages Manufacturers generally classify their bipolar

junction transistors into three broad categories

General Purpose / Small-signal Devices

Power Devices

RF (Radio Frequency / Microwave) Devices

Three categories of transistor packaging

Low Power

Medium Power

High Power

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Reference

The content of this lecture presentation was

complied from the following reference text

Electronic Devices 5th edition

FLOYD

Prentice Hall