Page 179 - Electrical Properties of Materials

Principles of semiconductor

9
devices

Les mystères partout coulent comme les sèves,
Baudelaire Les Sept Vieillards
This thing with knobs and a pretty light.
A. Wesker Chips with everything

9.1 Introduction
You have bravely endured lengthy discussions on rather abstract and
occasionally nebulous concepts in the hope that something more relevant to
the practice of engineering will emerge. Well, here we are; at last we are go-
ing to discuss various semiconductor devices. It is impossible to include all
of them, for there are so many nowadays. But if you follow carefully (and if
everything we have discussed so far is at your fingertips) you will stand a good
chance of understanding the operation of all existing devices and—I would
add—you should be in a very good position to understand the operation of
semiconductor devices to come in the near future. This is because human in-
genuity has rather narrow limits. Hardly anyone ever produces a new idea. It
is always some combination of old ideas that leads to reward. Revolutions are
few and far between. It is steady progress that counts.

9.2 The p–n junction in equilibrium
Not unexpectedly, when we want to produce a device, we have to put things
together. This is how we get the simplest semiconductor device the p–n junc-
tion, which consists of a p- and an n-type material in contact [Fig. 9.1(a)]. Let
∗
us imagine now that we literally put the two pieces together. What happens ∗ This is not how junctions are made.
when they come into contact? Remember, in the n-type material there are lots
of electrons, and holes abound in the p-type material. At the moment of contact
the electrons will rush over into the p-type material and the holes into the n-
type material. The reason is, of course, diffusion: both carriers make an attempt
to occupy uniformly the space available. Some electrons, moving towards the
left, collide head-on with the onrushing holes and recombine, but others will
be able to penetrate farther into the p-type material. How far? Not very far; or,
to put it another way, not many get very far because their efforts are frustrated
by the appearance of an electric field. The electrons leave positively charged
donor atoms behind, and similarly there are negatively charged acceptor atoms