bei48482_FM

368 Chapter Ten

to incorporate large numbers of them in an integrated circuit and they consume rel-

atively large amounts of power. The field-effect transistor(FET) lacks these dis-

advantages and is widely used today although slower in operation than junction

transistors.

As in Fig. 10.38, an n-channelFET consists of a block of n-type material with con-

tacts at each end together with a strip of p-type material on one side that is called the

gate.When connected as shown, electrons move from the sourceterminal to the

drainterminal through the n-type channel. The p-njunction is given a reverse bias,

and as a result both the nand pmaterials near the junction are depleted of charge

carriers (see Fig. 10.30b). The higher the reverse potential on the gate, the larger the

(b)

eF

eF

eF V 1 e

V 2 e

(a)

Emitter Base Collector

Conduction band

Forbidden band

Valence band

Lightly doped

n region

Heavily doped

n region

Lightly

doped p

region

eF

Figure 10.37(a) Isolated n-p-ntransistor. (b) Transistor connected as in Fig. 10.36. The forward bias

V 1 between emitter and base is small; the reverse bias V 2 between base and collector is large. Because

the base is very thin, electrons can pass through it from emitter to collector without recombining with

holes there. Once the electrons are in the collector, they undergo collisions in which they lose energy,

and afterward cannot return to the base because the potential hill V 2 eis too high.

Input

signal

Source

n-type channel

Depletion region

Drain

Output

load

p-type gate

+

Figure 10.38A field-effect transistor.

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