Page 451 - Schaum's Outline of Theory and Problems of Applied Physics

436 THE SOLID STATE [CHAP. 35

Fig. 35-9

SOLVED PROBLEM 35.9
Explain the operation of a semiconductor diode.
The operation of a semiconductor diode is based on the properties of a junction between p- and n-type materials.
In the diode shown in Fig. 35-10, the left-hand end is a p-type region in which conduction occurs by the motion
of holes, and the right-hand end is an n-type region in which conduction occurs by the motion of electrons. In
Fig. 35-10(b), a voltage is applied across the crystal so that the p end is negative and the n end is positive. This
situation is called reverse bias. The holes in the p region migrate to the left and disappear at the negative terminal,
while the electrons in the n end migrate to the right and disappear at the positive terminal. New electron-hole pairs
are spontaneously created by thermal excitation, but they are few in number and the resulting current is extremely
small.

p region n region
p-n junction
− +
electron
Holes Electrons
hole disappear I disappear

(a) (b)

Electrons and holes
recombine at junction
+ −

New holes New electrons
created added
I
(c)
Fig. 35-10

Figure 35-10(c) shows the same crystal with a forward bias: The p end is now positive, and the n end is now
negative. In this case new holes are created continuously by the removal of electrons at the positive terminal, and
new electrons are added at the negative terminal. The holes migrate to the right and the electrons to the left under
the influence of the applied voltage. The holes and electrons meet at the p-n junction and recombine there. Thus
current can flow readily in one direction through a p-n junction but hardly at all in the other, which makes a diode
of this kind an ideal rectifier to change an alternating current to a direct current.

SOLVED PROBLEM 35.10
Explain the operation of a light-emitting diode (LED).
Energy is needed to create an electron-hole pair, and this energy is given up when an electron and a hole
recombine. In silicon and germanium, the recombination energy is absorbed by the crystal as heat, but in certain
other semiconductors, such as gallium arsenide, a photon is emitted when recombination occurs. This is the basis of
the light-emitting diode.