Semiconductors are good examples of the theoretical and practical applications of modern electron theory. Their electrical resistivities vary in the range from about 10⁻² to 10⁹ Ω · cm. This range is intermediate between those of good conductors (10⁻⁶ to 10⁻⁵ Ω · cm) and insulators (10¹⁴ to 10²² Ω · cm). The elements and compounds discussed here are insulators at 0 K. These materials are covalently bonded. Their numbers of nearest neighbors are given by the (8 − N) rule, where N is the sum of their outermost s and p electrons. The compound semiconductors consist mainly of combinations of ions from Groups III and V and II and VI, but many other covalently bonded compounds also have desirable properties. These materials are intrinsic semiconductors in the “pure” state. They have negative temperature coefficients of electrical resistivity.