Conductors, Insulators, Semiconductors
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.
Insofar as transistor operation is concerned, only the loosely bound orbital electrons and their associated protons are of importance.