A crystalline solid with an electrical conductivity (typically 105–10−7 siemens per metre) intermediate between that of a conductor (up to 109 S m−1) and an insulator (as low as 10−15 S m−1). Semiconducting properties are a feature of metalloid elements, such as silicon and germanium. As the atoms in a crystalline solid are close together, the orbitals of their electrons overlap and their individual energy levels are spread out into energy bands. Conduction occurs in semiconductors as the result of a net movement, under the influence of an electric field, of electrons in the conduction band and empty states, called holes, in the valence band. A hole behaves as if it was an electron with a positive charge. Electrons and holes are known as the charge carriers in a semiconductor. The type of charge carrier that predominates in a particular region or material is called the majority carrier and that with the lower concentration is the minority carrier. An intrinsic semiconductor is one in which the concentration of charge carriers is a characteristic of the material itself; electrons jump to the conduction band from the valence band as a result of thermal excitation, each electron that makes the jump leaving behind a hole in the valence band. Therefore, in an intrinsic semiconductor the charge carriers are equally divided between electrons and holes. In extrinsic semiconductors the type of conduction that predominates depends on the number and valence of the impurity atoms present. Germanium and silicon atoms have a valence of four. If impurity atoms with a valence of five, such as arsenic, antimony, or phosphorus, are added to the lattice, there will be an extra electron per atom available for conduction, i.e. one that is not required to pair with the four valence electrons of the germanium or silicon. Thus extrinsic semiconductors doped with atoms of valence five give rise to crystals with electrons as majority carriers, the so-called n-type conductors. Similarly, if the impurity atoms have a valence of three, such as boron, aluminium, indium, or gallium, one hole per atom is created by an unsatisfied bond. The majority carriers are therefore holes, i.e. p-type conductors.
Subjects: Physics — Chemistry.