Metal-insulator transitions

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Abstract
The valence and conduction bands in semi-conductors normally owe their properties to their crystal structures. Sometimes changes in pressure or composition in mixed crystals can lead to an overlap between these bands, which results in a transition to metallic behaviour. Repulsion between electrons can also lead to the formation of two bands, which may or may not overlap; again metal-insulator transitions can occur. The most important and controversial question about these transitions is whether they are accompanied by a discontinuous change in the density of current carriers; the evidence for and against this is described. Applications are to vanadium trioxide and highly doped silicon and germanium.