Semimetals and narrow gap semiconductors

The characteristic feature of semimetals is that the conduction band edge is slightly lower than that of the valence band; this small band overlap leads to a small, equal concentration of electrons and holes. The Fermi surfaces are small, carrier effective masses low and mobilities high. In the closely related narrow gap semioonductors the band edges are close but do not overlap. The electronic properties of both types of material are determined by carriers in states which lie close to the band edges. Often it is necessary to tailor the energy gap for a particular device application; one way of doing this is by alloying: the effects of alloying on the energy gap are discussed.