transistor invention 1947 › sd › files › 2011 › 02 › transistor_en.pdf · it is an active...

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EE141© 2010, Associate Professor PhD. T.Vasileva

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Bipolar Junction Transistor

Semiconductor

Elements


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Transistor Invention – 1947

William Shockley,

Walter Brittain, and John Bardeen

Winners of the 1956 Nobel Prize in Physics

First Point Contact Transistor

AT&T Bell Laboratories

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Advantages

Bipolar transistors effectively replace the huge vacuum tubes.

Their advantages are:

solid body and small size;

low heat generation;

relatively small power requirements.

This makes the miniaturization of complex circuitry possible.


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Basic Feature

It is an active semiconductor device - element that supplies the rest of the

circuit with energy. It allows a small signal to control a much larger, high

powered one.

The Bipolar Junction Transistor (BJT) is a solid-state device for amplifying,

controlling, and generating electrical signals.

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5

Transistor Structure

The bipolar transistor has three regions: the emitter, the base, and

the collector;

The emitter is heavily doped and emits free charge carriers.

The base controls the flow of charges. It is very thin.

The collector collects the charge carriers from the base.

Emitter

Base

Collector

Emitter junction Collector junction


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Types and Schematic Symbols

Two types of bipolar transistors exist - NPN and PNP. Both transistor

types operate the same way but have opposite voltage polarities and

currents directions.

The figure shows the schematic symbols for a transistor and the relation

between the symbol electrodes and the transistor's structure. The arrow

indicates that this electrode is the emitter.

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Transistor's Mode of OperationA transistor's mode of operation depends on the various conditions possible at

both pn junctions. They are caused by the polarity of the applied voltage at

these junctions. Four transistor’s regimes are possible:

Forward-active mode

emmiter junction forward biased

collector junction reverse biased

Cut off mode

emmiter junction reverse biased

collector junction reverse biased

Saturation mode

emmiter junction forward biased

collector junction forward biased

Inverse-active mode

emmiter junction reverse biased

collector junction forward biased

E C

B B

Emmiter

Junction

Voltage

Collector

Junction

Voltage


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There are three transistor configurations depending on which electrode is common

to both the input and output circuits.

Common-base

connection

Common-emitter

connectionCommon-collector

connection

Input Output

Input

Output

Input

Output

The left side of the circuit is called the input circuit, and the right side is called

the output circuit.

The circuit arrangement in the first figure is known as a common-base configuration

because the base is common to both the emitter and collector circuits.

Transistor Connections

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Biased Transistor – CB Connection

The emitter-base junction is forward biased and the collector-base junction

is reverse biased. This mode of operation is called the active mode.

For a PNP transistor, all voltage polarities are the opposite.

The bipolar junction transistor is a normally off device. It needs dc voltages

applied to both pn junctions to start conducting.


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Principle of Operation

The principle of operation of the bipolar transistors is based on processes

occurring in two closely spaced and influenced each other pn junctions.

The name "transistor" is derived from "trans resistor", meaning that it can

transfer its internal resistance from low R in the forward biased emitter-base

circuit to a much higher R in the reverse biased collector-base circuit.

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Physical Processes in Emitter

If VEE is greater than the barrier potential, the emitter emits or injects electrons into the

base. Since the emitter is doped more heavily than the base the forward current at the

emitter-base junction is carried primarily by electrons.

IE

InE

Injection coefficient1E

nE

I

I


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Physical Processes in Base&Collector

Electrons entering the P base are the minority carriers in this region. Since the

base is lightly doped and very thin, a very small number of electrons recombine

with holes in the base. Most of the free electrons move on to the collector junction.

They appear as extra minority carriers there and are extracted into the collector by the

reverse collector voltage. As a result, almost all electrons supplied by the emitter flow

in the collector circuit.

InC

1nE

nC

I

I Transfer coefficient

InE

nCC MII

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Transistor Currents

There are three different currents in a transistor:

The emitter current is the largest current because it is the source of free charges.

EnEnCnCC IIMIMMII M

The collector current approximately equals the emitter current but less than it.

The base current is the smallest one. IB is measured in microamperes.

EC II


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Currents Relations in CB Connections

0CBEC III BCE III

10

E

C

E

CBC

I

I

I

II

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Common Emitter Connection

000 )( CBBCCBBCCBEC IIIIIIIII

0CBEC III

BCE III

Principle of operation of BJT does

not depend of it’s circuit connection.

0)1( CBBC III 0

)1(

1

)1(CBBC III

1 0)1( CBBC III

000 )( CBBCCBBCCBEC IIIIIIIII


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Currents Relations in CE Connection

0CBEC III BCE III

0)1( CBBC III 00 )1( CBCE II If IB=0, IC = ICE0

0CEBC III 10

B

C

B

CBC

I

I

I

II

The ratio of the collector current to the base current is called the current gain,

symbolized as βdc or hFE.

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Examples

β = ? IC = ? IE = ?


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The Currect Gain

In response to a small change in the base current a large change in collector

current could appear. This is called current amplification, a very useful

circuit property. Common-emitter configuration provides both current and

voltage amplification and hence has high power amplification factor.

The CE current gain has a high

value since the collector current

is much larger than the base

current.

For low-power transistor, the βdc

is typically 100 to 300.

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The Currect Gain Variation

The current gain has a wide variation with collector current, temperature

change and transistor replacement. Because of manufacturing tolerances,

the current gain of a transistor may vary over as much as 3:1 range with

transistor replacement of the same transistor type.


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Transistor Curents and VoltagesIB

IC

UBE

UEC

BBBEBB RIUE

CCCCCE RIEU

C

CECCC

R

UEI

B

BEBBB

R

UEI

BCE III

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ExamplesIB

IB = ? UCE = ?RC, RB = ?

so that UCE = 7.5V

UCE = ? PC =?PC =?


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VA Characteristics

Uin Uout

Iin Iout

Output

Characteristics

Input

Characteristics

Transfer

Characteristics

Characteristics of

voltage feedback

Iout

Uout

Iin = constUout = const

Iin = constUout = const

Iin

Uin

There are internal relationship

between input and output

voltages and currents in BJT.

Figure shows four families of VA

characteristics. The most important

are output and input characteristics.

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CB – Output Characteristics

Uin = UEBUout = UCB

Iout = IcIin = IEForward-active mode

Saturation

Cut-off (IE = 0, IC = ICB0)

IE = const

IC = f (UCB)IE= const

C

CBCBEC

r

UIII 0 α=f (IE)

C

CBC

dI

dUr

IE = const


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CB – Input Characteristics

Uin = UEBUout = UCB

Iout = IcIin = IE

IE = f (UEB)UCB=const

E

EB

E

EBin

I

U

dI

dUr

UCB = const

ΔIE

ΔUEB

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CE – Output Characteristics

Uout = UCE

Uin = UBE

Forward-active mode

Cut-off (IB = 0, IC = ICE0)

Saturation

IC = f (UCE)IB = const

*0

C

CECEBC

r

UIII β = f (IC)

1

* C

C

CEC

r

dI

dUr

IB

IC

ΔUCE

ΔIC


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CE – Input Characteristics

Uout = UCE

Uin = UBE

IB = f (UBE)UCE = const

UCE= 0VUCE= 5V

ΔIB

ΔUBE

B

BE

B

BEin

I

U

dI

dUr

IB

IC

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Influence of Temperature

BJT in CE connection is more temperature depended than in CB connection

since ICE0 and β increases faster with temperature than ICB0 and α.

ICE0 = (1+β)ICB0β = f(T)


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Rathings

Maximum ratings are the limits on the transistor currents, voltages, powers

and other quantities.

These parameters normally represent a destructive level that a designer

usually avoids under all operating conditions. For reliable device operation

maximum ratings should not even be approached. Otherwise, the device may

cease to function properly or its useful lifetime may be greatly shortened.

Maximum ratings for either type of transistor are given in data sheets.

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Max Power Max junction temperature TCmax < Ti, where n=p=ni

Max collector power PCmax

th

aCC

R

TTP

max

max

th

aC

R

TTP

P = UCIC Power dissipated in

collector pn junction

Power conducted

into the ambient

surroundings

th

aC

R

TTUI

The power dissipation must always be

less than the max power PCmax. Otherwise

the transistor will be destroyed.

When power is dissipated within a device, its junction

temperature TC tends to rise. The higher the power, the

higher the junction temperature.


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Bipolar Transistor Packages

Large power transistors have the collector connected to

the metal case to remove heat as easy as possible.

The power rating can be increased if the internal heat

is dissipated faster.

Bipolar transistors are available with plastic or metal cases.

When the power increases a metal tab provides a path out

of the transistor for heat.

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Heat Removal — Thermal Resistance

jcca thth RR cajc ththth RRR

hachjc thththth RRRR

Thermal energy can be easily reduced through conduction and radiation from

the device's case.

Heat sink

Thermal resistance Rth indicates efficiency in removing heat from the transistor

in units oK/W.

The less thermal resistance the higher power rating.

th

aCC

R

TTP

max

max


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Max Current

th

aCCCCE

R

TTPIU

max

maxmax

The maximum collector current of a bipolar transistor ICmax gives the

maximum current a bipolar transistor can handle without exceeding its

power rating PCmax.

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Reverse Breakdown Ratings

0CEBRU

0CBBRU

represents the voltage from collector to emitter with the base

open. This is the breakdown voltage in CE connection.

represents the voltage from collector to base with the emitter

open. This is the breakdown voltage in CB connection.

For normal transistor operation voltages should always be less than the

breakdown values.

000 EBCECB BRBRBR UUU


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Safty Operation Area

ICmax

PCmax

UCmax< UBR

SOA

If the BJT works in a SOA there

is a guarantee that it never will

exceed transistor max rathings.

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Transistor Testing

The emitter-base junction for a normal transistor should have a low resistance in one

direction and a high resistance in the other. The collector junction is tested the same way.

An ohmmeter can be used to check a PN junction either for an open circuit or a

short circuit. A good diode has very high ratio of reverse to forward resistance. For

testing bipolar transistors, the same diode test can be used for each PN junction.

transistor invention 1947 › sd › files › 2011 › 02 › transistor_en.pdf · it is an active...

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