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1

Chapter 4 Physics of Bipolar Transistors

4.1 General Considerations

4.2 Structure of Bipolar Transistor

4.3 Operation of Bipolar Transistor inActive Mode

4.4 Bipolar Transistor Models

4.5 Operation of Bipolar Transistor inSaturation Mode

4.6 The PNP Transistor 

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CH4 Physics of Bipolar Transistors 2

Bipolar Transistor 

In the chapter, we will study the physics of bipolartransistor and derive large and small signal models.

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3

Structure and Symbol of Bipolar Transistor 

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4

Structure and Symbol of Bipolar Transistor 

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CH4 Physics of Bipolar Transistors 5

Structure and Symbol of Bipolar Transistor 

Bipolar transistor can be thought of as a sandwich of threedoped Si regions. The outer two regions are doped with thesame polarity, while the middle region is doped withopposite polarity.

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6

Structure and Symbol of Bipolar Transistor 

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CH4 Physics of Bipolar Transistors 7

Injection of Carriers

Reverse biased PN junction creates a large electric fieldthat sweeps any injected minority carriers to their majorityregion.

This ability proves essential in the proper operation of abipolar transistor.

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CH4 Physics of Bipolar Transistors 8

Forward Active Region

Forward active region: VBE > 0, VBC < 0.

Figure b) presents a wrong way of modeling figure a).

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CH4 Physics of Bipolar Transistors 9

Emitter Current

Applying Kirchoff’s current law to the transistor, we can

easily find the emitter current.

 

  

 

  

  

  

11

C  E 

C 

 B

 B

C 

 BC  E 

 I  I 

 I  I 

 I 

 I 

 I  I  I 

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CH4 Physics of Bipolar Transistors 10

Summary of Currents

     

  

  

  

1

exp1

exp

1

exp

T 

 BE 

S  E 

T 

 BE 

S  B

T 

 BE 

S C 

V  

V   I  I 

V  

V   I  I 

V  

V   I  I 

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11

IO Characteristic

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CH4 Physics of Bipolar Transistors 12

Constant Current Source

Ideally, the collector current does not depend on thecollector to emitter voltage. This property allows thetransistor to behave as a constant current source when itsbase-emitter voltage is fixed.

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CH4 Physics of Bipolar Transistors 13

Bipolar Transistor Large Signal Model

A diode is placed between base and emitter and a voltage

controlled current source is placed between the collector

and emitter.

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CH4 Physics of Bipolar Transistors 14

Characteristics of Bipolar Transistor

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CH4 Physics of Bipolar Transistors 15

Example: IV Characteristics

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CH4 Physics of Bipolar Transistors 16

Simple Transistor Configuration

Although a transistor is a voltage to current converter,output voltage can be obtained by inserting a load resistorat the output and allowing the controlled current to passthru it.

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CH4 Physics of Bipolar Transistors 17

Example: Maximum RL

As RL increases, Vx drops and eventually forward biases thecollector-base junction. This will force the transistor out offorward active region.

Therefore, there exists a maximum tolerable collectorresistance.

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CH4 Physics of Bipolar Transistors 18

Transconductance

Transconductance, gm shows a measure of how well thetransistor converts voltage to current.

It will later be shown that gm is one of the most importantparameters in circuit design.

T 

C m

T 

 BE 

S T 

m

T 

 BE S 

 BE 

m

V 

 I  g 

V 

V  I 

V  g 

V 

V  I 

dV 

d  g 

 

  

 

exp1

exp

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CH4 Physics of Bipolar Transistors 19

Visualization of Transconductance

gm can be visualized as the slope of IC versus VBE.

A large IC has a large slope and therefore a large gm.

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CH4 Physics of Bipolar Transistors 20

Transconductance and Area

When the area of a transistor is increased by n, IS increasesby n. For a constant VBE, IC and hence gm increases by a

factor of n.

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CH4 Physics of Bipolar Transistors 21

Transconductance and Ic

The figure above shows that for a given VBE swing, thecurrent excursion around IC2 is larger than it would bearound IC1. This is because gm is larger IC2.

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CH4 Physics of Bipolar Transistors 22

Small-Signal Model: Derivation

Small signal model is derived by perturbing voltagedifference every two terminals while fixing the third terminal

and analyzing the change in current of all three terminals.

We then represent these changes with controlled sources

or resistors.

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CH4 Physics of Bipolar Transistors 23

Small-Signal Model: VBE Change

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CH4 Physics of Bipolar Transistors 24

Small-Signal Model: VCE Change

Ideally, VCE has no effect on the collector current. Thus, it

will not contribute to the small signal model.

It can be shown that VCB has no effect on the small signal

model, either.

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CH4 Physics of Bipolar Transistors 25

Small Signal Example I

Here, small signal parameters are calculated from DC

operating point and are used to calculate the change in

collector current due to a change in VBE.

375

75.3

1

m

T 

C 

m

 g 

r 

V 

 I  g 

   

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CH4 Physics of Bipolar Transistors 26

Small Signal Example II

In this example, a resistor is placed between the powersupply and collector, therefore, providing an outputvoltage.

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CH4 Physics of Bipolar Transistors 27

AC Ground

Since the power supply voltage does not vary with

time, it is regarded as a ground in small-signal

analysis.

(pada small signal analysis, Catu daya (VDD/VCC)

dianggap sebagai ground /diubah menjadi ground)

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CH4 Physics of Bipolar Transistors 28

Early Effect

The claim that collector current does not depend on VCE isnot accurate.

As VCE increases, the depletion region between base andcollector increases. Therefore, the effective base widthdecreases, which leads to an increase in the collectorcurrent.

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CH4 Physics of Bipolar Transistors 29

Early Effect Illustration

With Early effect, collector current becomes larger than

usual and a function of VCE.

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CH4 Physics of Bipolar Transistors 30

Early Effect

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CH4 Physics of Bipolar Transistors 31

Early Effect Representation

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CH4 Physics of Bipolar Transistors 32

Early Effect and Large-Signal Model

Early effect can be accounted for in large-signal model by

simply changing the collector current with a correction

factor.

In this mode, base current does not change.

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CH4 Physics of Bipolar Transistors 33

Early Effect and Small-Signal Model

C 

 A

T 

 BE 

S 

 A

C 

CE 

o

 I 

V 

V 

V  I 

V 

 I 

V r   

exp

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CH4 Physics of Bipolar Transistors 34

Summary of Ideas

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CH4 Physics of Bipolar Transistors 35

Overall I/V Characteristics

The speed of the BJT also drops in saturation.

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CH4 Physics of Bipolar Transistors 36

Bipolar Transistor in Saturation

When collector voltage drops below base voltage and

forward biases the collector-base junction, base current

increases and decreases the current gain factor, .

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CH4 Physics of Bipolar Transistors 37

Example: Acceptable VCC Region

In order to keep BJT at least in soft saturation region, thecollector voltage must not fall below the base voltage by

more than 400mV.

A linear relationship can be derived for VCC and RC and an

acceptable region can be chosen.

)400(   mV V  R I V  BE C C CC 

 

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CH4 Physics of Bipolar Transistors 38

Deep Saturation

In deep saturation region, the transistor loses its voltage-

controlled current capability and VCE becomes constant.

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CH4 Physics of Bipolar Transistors 39

A Comparison between NPN and PNP Transistors

The figure above summarizes the direction of current flowand operation regions for both the NPN and PNP BJT’s.

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CH4 Physics of Bipolar Transistors 40

PNP Equations

 

  

 

 

  

 

 A

 EC 

T 

 EB

S C 

T 

 EB

S  E 

T 

 EBS 

 B

T 

 EB

S C 

V 

V 

V 

V  I  I 

V 

V  I  I 

V 

V  I  I 

V 

V  I  I 

1exp

exp1

exp

exp

  

    

 Early Effect 

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CH4 Physics of Bipolar Transistors 41

Large Signal Model for PNP

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CH4 Physics of Bipolar Transistors 42

PNP Biasing

Note that the emitter is at a higher potential than both the

base and collector.

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CH4 Physics of Bipolar Transistors 43

Small Signal Analysis

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CH4 Physics of Bipolar Transistors 44

Small-Signal Model for PNP Transistor 

The small signal model for PNP transistor is exactly

IDENTICAL to that of NPN. This is not a mistake because

the current direction is taken care of by the polarity of VBE.

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CH4 Physics of Bipolar Transistors 45

Small Signal Model Example I

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CH4 Physics of Bipolar Transistors 46

Small Signal Model Example II

Small-signal model is identical to the previous ones.

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CH4 Physics of Bipolar Transistors 47

Small Signal Model Example III

Since during small-signal analysis, a constant voltage

supply is considered to be AC ground, the final small-signal

model is identical to the previous two.

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Small Signal Model Example IV