Conductivity in semiconductors

Semiconductors (silicon, germanium, selenium, tellurium) lie between metals and dielectrics in conductivity. Unlike metals, their resistivity falls when they are heated or illuminated; at very low temperature they behave like dielectrics. In a pure silicon crystal each atom is bonded covalently to its neighbours by four valence electrons. On heating some bonds break: an electron becomes free (electron conductivity, n-type) and leaves a positively charged hole; a neighbouring electron can fill the hole and so move it (hole conductivity, p-type). In a pure (undoped) semiconductor the numbers of electrons and holes are equal – intrinsic conductivity, which is small. Impurities raise the conductivity sharply. A five-valent atom (arsenic, phosphorus) is a donor: it gives one extra free electron, the main carriers are electrons – n-type. A three-valent atom (indium, boron, gallium) is an acceptor: one bond remains incomplete, i.e. a hole appears, and the main carriers are holes – p-type. The minority carriers are holes in n-type and electrons in p-type.