bei48482_FM

366 Chapter Ten

I

Tunneling

current

a V

b

c

Figure 10.35Voltage-current characteristic of a Zener diode.

Figure 10.34Voltage-current

characteristic of a tunnel diode.

The points a, b, and ccorrespond

to parts a, b, and cof Fig. 10.33.

The dashed line indicates the be-

havior of an ordinary junction

diode, as in Fig. 10.30.

I

Reverse bias Forward bias

V

Conduction

band

p region n region

(a)

Valence band

(b)

eF eF

+ –

eF

(c)

eF

+ –

eF

Figure 10.33Operation of a tunnel diode. (a) No bias. Electrons tunnel both ways between the p

and nregions. (b) Small forward bias. Electrons tunnel from the nto the pregion only. (c) Larger

forward bias. Now the valence band of the pregion does not overlap the conduction band of the

nregion and so no tunneling can occur. At higher voltages the diode behaves like the ordinary

diode of Fig. 10.30.

The importance of the tunnel diode lies in the rapidity with which a voltage change

between aand bor between band cin Fig. 10.34 can alter the current. In ordinary

diodes and transistors, the response time depends on the diffusion speed of the charge

carriers, which is low. Hence such devices operate slowly. Tunnel diodes, on the other

hand, respond quickly to appropriate voltage changes and can be used in high-

frequency oscillators and as fast switches in computers.

Zener Diode

Although the reverse current in many semiconductor diodes remains virtually constant

even at high voltages, as in Fig. 10.31, in certain diodes the reverse current increases

abruptly when a particular voltage is reached, as in Fig. 10.35. Such diodes are called

Zener diodesand are widely used in voltage-regulation circuits.

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