Review lecture: Metal–insulator transitions

Abstract
A survey is given of a variety of solids that show a metal–insulator transition. In crystals most transitions are expected to be of first order as the composition or temperature is changed; in disordered systems this is not necessarily the case. The transition in an impurity band with change of donor concentration is described, and also with change of stress, magnetic field or gate voltage. The concept of a minimum metallic conductivity is discussed, with special reference to materials of mixed valence.