|A material whose electrical conductivity increases with increasing temperature (this is in contrast to conductors, whose conductivity decreases). The distinction between semiconductors and insulators is less clear, as it depends on the magnitude of the energy gap between the conduction band and the valence band. Insulators have a large gap, whereas semiconductors have a small one. Semiconductors fall into two categories: intrinsic and extrinsic. Intrinsic semiconductors, such as pure silicon or germanium, operate by thermal excitation taking an electron from the valence band to the conduction band, the electron excited plus the hole left behind causing conduction. In extrinsic semiconductors, added impurities (dopants) provide additional energy levels between the valence and conduction bands which allow either electrons (n-type) or holes (p-type) to be the majority carriers. Impurities used include elements from Group 15, such as phosphorus and arsenic, which produce n-type behaviour, and elements from Group 13, such as aluminium and gallium, which produce p-type behaviour. While silicon and germanium are the most important semiconducting materials, compounds such as gallium arsenide (GaAs) are becoming increasingly useful. A third isoelectronic series consists of compounds such as lead(II) telluride (PbTe). The p-n junction assumes central importance in electronic circuits.See diode; inhomogeneous semiconductor; semimetal; transistor.