Solid-State Devices
Bipolar Junction Transistors
‘N’- and ‘P’-Type Materials
Most materials can be grouped in one or other of three classes, insulators, semiconductors, or conductors, depending on the ease or difficulty with which electrons can pass through them. In insulators, all of the electrons associated with the atomic structure will be firmly bound in the valency bands of the material, whereas in good, usually metallic, conductors many of the atomic electrons will only be loosely bound and will be free to move within the body of the material.
In semiconductors, at temperatures above absolute zero (0°K or –273.15°C), electrons will exist both in the valency levels where they are not free to leave the atoms with which they are associated and in the conduction band in which they are free to travel within
the body of the material. This characteristic is influenced greatly by the “doping” of the material, which is normally done during the manufacture of the semiconductor material by introducing carefully controlled amounts of specific impurities into the molten mass from which the single semiconductor crystal is grown. The most common semiconductor material in normal use is silicon because it is inexpensive, readily available, and has good thermal properties. Germanium, the material from which all early transistors were made, has electrical characteristics that are influenced greatly by its temperature, which
is inconvenient in use. Also, it does not lend itself at all well to contemporary mass- production techniques.
In the case of silicon, which has very little conductivity in its undoped “intrinsic” form, the most common dopants are boron or aluminium, which give rise to a semiconductor with a deficiency of valency electrons, usually referred to as holes—called a ‘P’-type material—or phosphorus, which will cause the silicon to have a surplus of valency electrons, which forces some of them into the conduction band. Such a semiconductor material would be termed ‘N’ type. Both P-type and N-type silicon can be quite highly conductive, depending on the doping levels used.
Fermi Levels
The electron energy distribution in single-crystal P- and N-type materials is shown in Figure 11.6, and the mean electron energy levels, known as the Fermi levels, are shown.