ABSTRACT
In this chapter, the author presents the basic principles of band theory of solids, and semiconductors in particular in such a way that the mathematical apparatus is used only in the cases where it helps to understand the physical picture. The principle of the electrical neutrality of the semiconductor, which states that the total charge of holes and positive donor ions must be equal to the total charge of the conduction electrons and the negative acceptor ions, leads to an important conclusion that independently on the degree of impurity of a semiconductor. The more free charges per unit of time are generated in a unit of the semiconductor volume by irradiation, the greater is the rate of their recombination. Because photoconductivity in an extrinsic semiconductor is the result of the change of the majority carrier concentration, its magnitude strongly depends on the dark (or thermal) concentration of these carriers.
