Bird J. Electrical Circuit Theory and Technology

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12 Transistors

At the end of this chapter you should be able to:

ž understand the structure of a bipolar junction transistor

ž understand transistor action for p-n-p and n-p-n types

ž draw the circuit diagram symbols for p-n-p and n-p-n

transistors

ž appreciate common-base, common-emitter and

common-collector transistor connections

ž interpret transistor characteristics

ž appreciate how the transistor is used as an ampliﬁer

ž determine the load line on transistor characteristics

ž estimate current, voltage and power gains from transistor

characteristics

ž understand thermal runaway in a transistor

12.1 The bipolar

junction transistor

The bipolar junction transistor consists of three regions of semiconductor

material. One type is called a p-n-p transistor, in which two regions of

p-type material sandwich a very thin layer of n-type material. A second

type is called an n-p-n transistor, in which two regions of n-type material

sandwich a very thin layer of p-type material. Both of these types of tran-

sistors consist of two p-n junctions placed very close to one another in

a back-to-back arrangement on a single piece of semiconductor mate-

rial. Diagrams depicting these two types of transistors are shown in

Figure 12.1.

The two p-type material regions of the p-n-p transistor are called the

emitter and collector and the n-type material is called the base. Similarly,

the two n-type material regions of the n-p-n transistor are called the

emitter and collector and the p-type material region is called the base, as

shown in Figure 12.1.

Transistors have three connecting leads and in operation an electrical

input to one pair of connections, say the emitter and base connections

can control the output from another pair, say the collector and emitter

connections. This type of operation is achieved by appropriately biasing

the two internal p-n junctions. When batteries and resistors are connected

to a p-n-p transistor, as shown in Figure 12.2(a), the base-emitter junction

is forward biased and the base-collector junction is reverse biased.

Similarly, an n-p-n transistor has its base-emitter junction forward

biased and its base-collector junction reverse biased when the batteries

are connected as shown in Figure 12.2(b).

146 Electrical Circuit Theory and Technology

Collector

Emitter

Base

Collector

Emitter

Base

n-type

material

n-type

material

p-type

material

p-type

material

p-n-p transistor n-p-n transistor

Figure 12.1

For a silicon p-n-p transistor, biased as shown in Figure 12.2(a), if the

base-emitter junction is considered on its own, it is forward biased and

a current ﬂows. This is depicted in Figure 12.3(a). For example, if R

is 1000 , the battery is 4.5V and the voltage drop across the junction

is taken as 0.7V, the current ﬂowing is given by 4.5  0.7/1000 D

3.8mA.

Emitter Base Collector Emitter Base Collector

Emitter

resistor

Load

resistor

Emitter

resistor

Load

resistor

−− +

−+− −+−+

pnp npn

(a) p-n-p transistor (b) n-p-n transistor

Figure 12.2

When the base-collector junction is considered on its own, as shown in

Figure 12.3(b), it is reverse biased and the collector current is something

less than 1

µA.

However, when both external circuits are connected to the transistor,

most of the 3.8mA of current ﬂowing in the emitter, which previously

ﬂowed from the base connection, now ﬂows out through the collector

connection due to transistor action.

Transistors 147

+− +−

4.5 V

= 1000 Ω

0.7 V

Emitter Base

ppnn

Base Collector

(a) (b)

Figure 12.3

12.2 Transistor action

In a p-n-p transistor, connected as shown in Figure 12.2(a), transistor

action is accounted for as follows:

(a) The majority carriers in the emitter p-type material are holes

(b) The base-emitter junction is forward biased to the majority carriers

and the holes cross the junction and appear in the base region

so although some electron-hole pairs are formed, many holes are left

in the base region

(d) The base-collector junction is reverse biased to electrons in the base

region and holes in the collector region, but forward biased to holes

in the base region; these holes are attracted by the negative potential

at the collector terminal

(e) A large proportion of the holes in the base region cross the base-

collector junction into the collector region, creating a collector cur-

rent; conventional current ﬂow is in the direction of hole movement.

The transistor action is shown diagrammatically in Figure 12.4. For tran-

sistors having very thin base regions, up to 99.5% of the holes leaving

the emitter cross the base collector junction.

In an n-p-n transistor, connected as shown in Figure 12.2(b), transistor

action is accounted for as follows:

(a) The majority carriers in the n-type emitter material are electrons

Emitter Base Collector

Holes

Figure 12.4

(b) The base-emitter junction is forward biased to these majority carriers

and electrons cross the junction and appear in the base region

some recombination with holes occurs but many electrons are left

in the base region

(d) The base-collector junction is reverse biased to holes in the base

region and electrons in the collector region, but is forward biased

148 Electrical Circuit Theory and Technology

to electrons in the base region; these electrons are attracted by the

positive potential at the collector terminal

(e) A large proportion of the electrons in the base region cross the

base collector junction into the collector region, creating a collector

current.

The transistor action is shown diagrammatically in Figure 12.5. As stated

in Section 12.1, conventional current ﬂow is taken to be in the direction

of hole ﬂow, that is, in the opposite direction to electron ﬂow, hence the

directions of the conventional current ﬂow are as shown in Figure 12.5.

For a p-n-p transistor, the base-collector junction is reverse biased for

majority carriers. However, a small leakage current, I

CBO

ﬂows from

the base to the collector due to thermally generated minority carriers

(electrons in the collector and holes in the base), being present.

Emitter Base Collector

Electrons

+−

Figure 12.5

The base-collector junction is forward biased to these minority carriers.

If a proportion, ˛, (having a value of up to 0.995 in modern transistors),

of the holes passing into the base from the emitter, pass through the base-

collector junction, then the various currents ﬂowing in a p-n-p transistor

are as shown in Figure 12.6(a).

Emitter Base Collector Emitter Base Collector

pn n npp

CBO

∝

(1−∝)

(a) (b)

Figure 12.6

Similarly, for an n-p-n transistor, the base-collector junction is reversed

biased for majority carriers, but a small leakage current, I

CBO

ﬂows from

the collector to the base due to thermally generated minority carriers

(holes in the collector and electrons in the base), being present. The

base-collector junction is forward biased to these minority carriers. If a

proportion, ˛, of the electrons passing through the base-emitter junction

also pass through the base-collector junction then the currents ﬂowing in

an n-p-n transistor are as shown in Figure 12.6(b).

Problem 1. With reference to a p-n-p transistor, explain brieﬂy

what is meant by the term transistor action and why a bipolar

junction transistor is so named.

For the transistor as depicted in Figure 12.4, the emitter is relatively

heavily doped with acceptor atoms (holes). When the emitter terminal

is made sufﬁciently positive with respect to the base, the base-emitter

junction is forward biased to the majority carriers. The majority carriers

are holes in the emitter and these drift from the emitter to the base. The

Transistors 149

base region is relatively lightly doped with donor atoms (electrons) and

although some electron-hole recombination’s take place, perhaps 0.5%,

most of the holes entering the base, do not combine with electrons.

The base-collector junction is reverse biased to electrons in the base

region, but forward biased to holes in the base region. Since the base is

very thin and now is packed with holes, these holes pass the base-emitter

junction towards the negative potential of the collector terminal. The

control of current from emitter to collector is largely independent of the

collector-base voltage and almost wholly governed by the emitter-base

voltage. The essence of transistor action is this current control by means

of the base-emitter voltage.

In a p-n-p transistor, holes in the emitter and collector regions are

majority carriers, but are minority carriers when in the base region. Also

thermally generated electrons in the emitter and collector regions are

minority carriers as are holes the base region. However, both majority

and minority carriers contribute towards the total current ﬂow (see

Figure 12.6(a)). It is because a transistor makes use of both types of

charge carriers (holes and electrons) that they are called bipolar. The

transistor also comprises two p-n junctions and for this reason it is a

junction transistor. Hence the name —bipolar junction transistor.

(0 V)

(−6 V)

(6 V)

(−0.6 V)

(0.6 V)

p-n-p transistor

n-p-n transistor

Figure 12.7

12.3 Transistor symbols

Symbols are used to represent p-n-p and n-p-n transistors in circuit dia-

grams and are as shown in Figure 12.7. The arrowhead drawn on the

emitter of the symbol is in the direction of conventional emitter current

(hole ﬂow). The potentials marked at the collector, base and emitter are

typical values for a silicon transistor having a potential difference of 6V

between its collector and its emitter.

The voltage of 0.6 V across the base and emitter is that required to

reduce the potential barrier and if it is raised slightly to, say, 0.62 V, it is

likely that the collector current will double to about 2 mA. Thus a small

change of voltage between the emitter and the base can give a relatively

large change of current in the emitter circuit; because of this, transistors

can be used as ampliﬁers.

12.4 Transistor

connections

There are three ways of connecting a transistor, depending on the use

to which it is being put. The ways are classiﬁed by the electrode that is

common to both the input and the output. They are called:

(a) common-base conﬁguration, shown in Figure 12.8(a)

(b) common-emitter conﬁguration, shown in Figure 12.8(b)

These conﬁgurations are for an n-p-n transistor. The current ﬂows shown

are all reversed for a p-n-p transistor.

150 Electrical Circuit Theory and Technology

Problem 2. The basic construction of an n-p-n transistor makes it

appear that the emitter and collector can be interchanged. Explain

why this is not usually done.

In principle, a bipolar junction transistor will work equally well with either

the emitter or collector acting as the emitter. However, the conventional

emitter current largely ﬂows from the collector through the base to the

emitter, hence the emitter region is far more heavily doped with donor

atoms (electrons) than the base is with acceptor atoms (holes). Also, the

base-collector junction is normally reverse biased and in general, doping

density increases the electric ﬁeld in the junction and so lowers the break-

down voltage. Thus, to achieve a high breakdown voltage, the collector

region is relatively lightly doped.

In addition, in most transistors, the method of production is to diffuse

acceptor and donor atoms onto the n-type semiconductor material, one

after the other, so that one overrides the other. When this is done, the

doping density in the base region is not uniform but decreases from emitter

to collector. This results in increasing the effectiveness of the transistor.

Thus, because of the doping densities in the three regions and the non-

uniform density in the base, the collector and emitter terminals of a tran-

sistor should not be interchanged when making transistor connections.

INPUT

OUTPUT

(a)

(b)

(c)

Figure 12.8

12.5 Transistor

characteristics

The effect of changing one or more of the various voltages and currents

associated with a transistor circuit can be shown graphically and these

graphs are called the characteristics of the transistor. As there are ﬁve

variables (collector, base and emitter currents and voltages across the

collector and base and emitter and base) and also three conﬁgurations,

many characteristics are possible. Some of the possible characteristics are

given below.

(a) Common-base conﬁguration

(i) Input characteristic. With reference to Figure 12.8(a), the input to

a common-base transistor is the emitter current, I

, and can be varied

by altering the base emitter voltage V

. The base-emitter junction is

essentially a forward biased junction diode, so as V

is varied, the current

ﬂowing is similar to that for a junction diode, as shown in Figure 12.9

for a silicon transistor. Figure 12.9 is called the input characteristic for

an n-p-n transistor having common-base conﬁguration. The variation of

the collector-base voltage V

has little effect on the characteristic. A

similar characteristic can be obtained for a p-n-p transistor, these having

reversed polarities.

0.2 0.4 0.6 −

Emitter base voltage (V)

−

Emitter current (mA)

Figure 12.9

(ii) Output characteristics. The value of the collector current I

is very

largely determined by the emitter current, I

. For a given value of I

the collector-base voltage, V

, can be varied and has little effect on the

value of I

.IfV

is made slightly negative, the collector no longer

Transistors 151

attracts the majority carriers leaving the emitter and I

falls rapidly to

zero. A family of curves for various values of I

are possible and some

of these are shown in Figure 12.10. Figure 12.10 is called the output

characteristics for an n-p-n transistor having common-base conﬁguration.

Similar characteristics can be obtained for a p-n-p transistor, these having

reversed polarities.

−20 2 4 6 8

= 30 mA

= 20 mA

= 10 mA

Collector current (mA)

Collector-base voltage (V)

Figure 12.10

(b) Common-emitter conﬁguration

(i) Input characteristic. In a common-emitter conﬁguration (see

Figure 12.8(b)), the base current is now the input current. As V

varied, the characteristic obtained is similar in shape to the input char-

acteristic for a common-base conﬁguration shown in Figure 12.9, but the

values of current are far less. With reference to Figure 12.6(a), as long

as the junctions are biased as described, the three currents I

, I

and I

keep the ratio 1 : ˛ : 1 ˛, whichever conﬁguration is adopted. Thus

the base current changes are much smaller than the corresponding emitter

current changes and the input characteristic for an n-p-n transistor is as

shown in Figure 12.11. A similar characteristic can be obtained for a

p-n-p transistor, these having reversed polarities.

(ii) Output characteristics. A family of curves can be obtained, depen-

ding on the value of base current I

and some of these for an n-p-n

transistor are shown in Figure 12.12. A similar set of characteristics can

be obtained for a p-n-p transistor, these having reversed polarities. These

characteristics differ from the common base output characteristics in two

ways:

the collector current reduces to zero without having to reverse the

collector voltage, and

0 0.2 0.4 0.6 0.8

Base-emitter voltage (V)

100

150

200

250

300

Base current (µA)

Figure 12.11

the characteristics slope upwards indicating a lower output resistance

(usually kilohms for a common-emitter conﬁguration compared with

megohms for a common-base conﬁguration).

Problem 3. With the aid of a circuit diagram, explain how the

input and output characteristics of an n-p-n transistor having a

common-base conﬁguration can be obtained.

A circuit diagram for obtaining the input and output characteristics

for an n-p-n transistor connected in common-base conﬁguration is shown

in Figure 12.13. The input characteristic can be obtained by varying R

which varies V

, and noting the corresponding values of I

. This is

repeated for various values of V

. It will be found that the input char-

acteristic is almost independent of V

and it is usual to give only one

characteristic, as shown in Figure 12.9.

0246810

Collector-emitter voltage (V)

Collector current (mA)

= 300 µA

= 250 µA

= 200 µA

= 150 µA

= 100 µA

= 50 µA

= 0

Figure 12.12

To obtain the output characteristics, as shown in Figure 12.10, I

set to a suitable value by adjusting R

. For various values of V

,setby

adjusting R

, I

is noted. This procedure is repeated for various values of

. To obtain the full characteristics, the polarity of battery V

hastobe

reversed to reduce I

to zero. This must be done very carefully or else

−

Figure 12.13

152 Electrical Circuit Theory and Technology

values of I

will rapidly increase in the reverse direction and burn out

the transistor.

12.6 The transistor as an

ampliﬁer

The amplifying properties of a transistor depend upon the fact that current

ﬂowing in a low-resistance circuit is transferred to a high-resistance circuit

with negligible change in magnitude. If the current then ﬂows through a

load resistance, a voltage is developed. This voltage can be many times

greater than the input voltage which caused the original current ﬂow.

(a) Common-base ampliﬁer

The basic circuit for a transistor is shown in Figure 12.14 where an n-

p-n transistor is biased with batteries b

and b

. A sinusoidal alternating

input signal,

, is placed in series with the input bias voltage, and a load

resistor, R

, is placed in series with the collector bias voltage. The input

signal is therefore the sinusoidal current i

resulting from the applica-

tion of the sinusoidal voltage

superimposed on the direct current I

established by the base-emitter voltage V

Figure 12.14

Let the signal voltage

be 100mV and the base-emitter circuit resis-

tance be 50. Then the emitter signal current will be 100/50 D 2mA.

Let the load resistance R

D 2.5k. About 0.99 of the emitter current will

ﬂow in R

. Hence the collector signal current will be about 0.99 ð2 D

1.98mA and the signal voltage across the load will be 2500 ð 1.98 ð

3

D 4.95V. Thus a signal voltage of 100 mV at the emitter has produ-

ced a voltage of 4950 mV across the load. The voltage ampliﬁcation or

gain is therefore 4950/100 D 49.5 times. This example illustrates the

action of a common-base ampliﬁer where the input signal is applied

between emitter and base and the output is taken from between collector

and base.

5mA

12 V

1kΩ

−

0.1 mA

base d.c.

bias

Collector

voltage

variations

Figure 12.15

(b) Common-emitter ampliﬁer

The basic circuit arrangement of a common-emitter ampliﬁer is shown

in Figure 12.15. Although two batteries are shown, it is more usual to

employ only one to supply all the necessary bias. The input signal is

applied between base and emitter, and the load resistor R

is connected

between collector and emitter. Let the base bias battery provide a voltage

which causes a base current I

of 0.1mA to ﬂow. This value of base

current determines the mean d.c. level upon which the a.c. input signal

will be superimposed. This is the d.c. base current operating point.

Let the static current gain of the transistor, ˛

, be 50. Since 0.1mA

is the steady base current, the collector current I

will be ˛

ð I

50 ð 0.1 D 5 mA. This current will ﬂow through the load resistor R

(D 1k), and there will be a steady voltage drop across R

given by

D 5 ð 10

3

ð 1000 D 5V. The voltage at the collector, V

, will

therefore be V

 I

D 12  5 D 7 V. This value of V

is the mean

(or quiescent) level about which the output signal voltage will swing alter-

nately positive and negative. This is the collector voltage d.c. operating

Transistors 153

(µA)

200

100

0 0.5 1.0

(V)

Figure 12.16

point. Both of these d.c. operating points can be pin-pointed on the

input and output characteristics of the transistor. Figure 12.16 shows

the I

characteristic with the operating point X positioned at I

100

µA, V

D 0.75V, say.

Figure 12.17 shows the I

characteristics, with the operating point

Y positioned at I

D 5mA,V

D 7V. It is usual to choose the operating

point Y somewhere near the centre of the graph.

(mA)

(V)

= 100µA

5 mA

mean

collector

current

51015

7 V mean collector

voltage

Figure 12.17

It is possible to remove the bias battery V

and obtain base bias from

the collector supply battery V

instead. The simplest way to do this is

to connect a bias resistor R

between the positive terminal of the V

supply and the base as shown in Figure 12.18. The resistor must be of

such a value that it allows 100

µA to ﬂow in the base-emitter diode.

Figure 12.18

For a silicon transistor, the voltage drop across the junction for forward

bias conditions is about 0.6 V. The voltage across R

must then be 12 

0.6 D 11.4V. Hence, the value of R

must be such that I

ð R

D 11.4V,

i.e.

11.4

100 ð 10

6

D 114k.

With the inclusion of the 1k load resistor, R

, a steady 5mA collector

current, and a collector-emitter voltage of 7 V, the d.c. conditions are

established.

An alternating input signal (

) can now be applied. In order not to

disturb the bias condition established at the base, the input must be fed to

the base by way of a capacitor C

. This will permit the alternating signal

to pass to the base but will prevent the passage of direct current. The

reactance of this capacitor must be such that it is very small compared

with the input resistance of the transistor. The circuit of the ampliﬁer is

now as shown in Figure 12.19. The a.c. conditions can now be determined.

−

= α

Figure 12.19

When an alternating signal voltage

is applied to the base via capacitor

the base current i

varies. When the input signal swings positive, the

base current increases; when the signal swings negative, the base current

decreases. The base current consists of two components: I

, the static base

bias established by R

,andi

, the signal current. The current variation

will in turn vary the collector current, i

. The relationship between i

and i

is given by i

D ˛

, where ˛

is the dynamic current gain of

the transistor and is not quite the same as the static current gain ˛

; the

difference is usually small enough to be insigniﬁcant.

The current through the load resistor R

also consists of two compo-

nents: I

, the static collector current, and i

, the signal current. As i

increases, so does i

and so does the voltage drop across R

. Hence, from

the circuit:

D V

 I

C i

R

The d.c. components of this equation, though necessary for the ampliﬁer

to operate at all, need not be considered when the a.c. signal conditions

are being examined. Hence, the signal voltage variation relationship is:

D˛

ð i

ð R

D i

154 Electrical Circuit Theory and Technology

the negative sign being added because v

decreases when i

increases and

vice versa. The signal output and input voltages are of opposite polarity,

i.e. a phase shift of 180

has occurred. So that the collector d.c. potential

is not passed on to the following stage, a second capacitor, C

, is added

as shown in Figure 12.19. This removes the direct component but permits

the signal voltage

D i

to pass to the output terminals.

12.7 The load line

The relationship between the collector-emitter voltage (V

) and collector

current (I

) is given by the equation: V

D V

 I

in terms of the

d.c. conditions. Since V

and R

are constant in any given circuit, this

represents the equation of a straight line which can be written in the

y D mx Cc form. Transposing V

D V

 I

for I

gives:

 V



D





i.e. I

D





which is of the straight line form y D mx C c; hence if I

is plotted

vertically and V

horizontally, then the gradient is given by 1/R



and the vertical axis intercept is V

A family of collector static characteristics drawn on such axes is shown

in Figure 12.12 on page 151, and so the line may be superimposed on

these as shown in Figure 12.20.

46810

Collector-emitter voltage (V)

Collector current (mA)

= 0

= 50 µA

= 100 µA

= 200 µA

= 250 µA

= 300 µA

LOAD LINE

−

Figure 12.20

The reason why this line is necessary is because the static curves relate

to V

for a series of ﬁxed values of I

. When a signal is applied to