Conductors, Insulators, Semiconductors

Author: Leonard Krugman

Conductors are materials that have a large number of loosely bound valence-ring electrons; these electrons are easily knocked out of their orbit and are then referred to as free electrons. Insulators are materials in which the valence-ring electrons are tightly bound to the nucleus. In between the limits of these two major categories is a third general class of materials called semiconductors. For example, transistor germanium, a semiconductor, has approximately one trillion times (1 × 10¹²) the conductivity of glass, an insulator, but has only about one thirty-millionth (3 × 10^-8) part of the conductivity of copper, a conductor.

The heart of the transistor is a semiconductor, generally the germanium crystal. Other semiconductors such as selenium and silicon have been used in transistors, but germanium has proved to be the most widely applicable material. The general semiconductor principles discussed in this book apply to all elements used as transistor semiconductors.

Insofar as transistor operation is concerned, only the loosely bound orbital electrons and their associated protons are of importance. For the purposes of future discussion it is therefore convenient to picture the carbon atom in the short form illustrated in Fig. 1-2. Note that in this figure only the valence-ring electrons and their associated protons are indicated; the tightly bound inner orbit electrons and their respective protons are not shown. Thus, the carbon atom in the short form contains a nucleus with a +4 charge around which the four valence-ring electrons rotate. The short form simplifies the graphical representation of semiconductor operation, as will be seen later.