Page 141 - Electrical Properties of Materials

Semiconductors

8

Whatsoever things are true ...
whatsoever things are pure ...
think on these things.
Philippians iv 8

8.1 Introduction
With the aid of band theory we have succeeded in classifying solids into metals,
insulators, and semiconductors. We are now going to consider semiconductors
(technologically the newest class) in more detail. Metals and insulators have
been used for at least as long as we have been civilized, but semiconductors
have found application only in the last century, and their more widespread
application dates from the 1950s. Over this period the electronics industry has
been (to use a hackneyed word justifiably) revolutionized, first by the transistor,
then by microelectronic circuitry. Each of these in succession, by making cir-
cuitry much cheaper and more compact, has led to the wider use of electronic
aids, such as computers, in a way that is revolutionary in the social sense too.
Perhaps the key reason for this sudden change has been the preparation of
extremely pure semiconductors, and hence the possibility of controlling im-
purity; this was a development of the 1940s and 1950s. By crystal pulling,
zone-refining, and epitaxial methods it is possible to prepare silicon and
10
germanium with an impurity of only 1 part in 10 . Compare this with
long-established engineering materials, such as steel, brass, or copper where
impurities of a few parts per million are still virtually unattainable (and for
most purposes, it must be admitted, not required). Probably the only other ma-
terial that has ever been prepared with purity comparable to that of silicon and
germanium is uranium, and we know what that led to.
I shall now try to show why the important electrical properties of semicon-
ductors occur and how they are influenced and controlled by small impurity
concentrations. Next, we shall consider what really came first, the prepara-
tion of pure material. We shall be ready then to discuss junction devices and
integrated circuit technology.

8.2 Intrinsic semiconductors

The aim in semiconductor technology is to purify the material as much as
possible and then to introduce impurities in a controlled manner. We shall call
the pure semiconductor ‘intrinsic’ because its behaviour is determined by its
intrinsic properties alone, and we shall call the semiconductor ‘extrinsic’ after
external interference has changed its inherent properties. In devices it is mostly
extrinsic semiconductors that are used, but it is better to approach our subject
gradually and discuss intrinsic semiconductors first.