Figure 1: Metal–insulator transitions can occur for different reasons, changing the band structure of the system. Take sodium, an alkali metal, as an example. It has one valence electron per ...

An insulator has a band structure in which occupied and empty bands are separated by an energy gap. The existence of an energy gap, however, does not guarantee that a material is a simple insulator.

At an atomic level, semiconductors have a crystalline structure in which their electrons are arranged in energy bands. The ...