Extrinsic Semiconductors: n-type and p-type

Why dope at all. An intrinsic semiconductor's conductivity (Section 9.7) is small and strongly temperature-dependent, making it, on its own, of limited practical use for building a reliable device. DELIBERATELY adding a tiny, carefully controlled trace of a chosen impurity atom -- called DOPING -- lets the type and concentration of the dominant charge carrier be engineered almost at will, and is the foundational idea behind every semiconductor device developed in the rest of this chapter.

n-type semiconductor (pentavalent doping). Doping a tetravalent host crystal (silicon or germanium) with a trace of a PENTAVALENT impurity atom (Group 15: phosphorus, arsenic, antimony -- five valence electrons) has this effect: the impurity atom takes up a regular lattice site, and FOUR of its five valence electrons form the usual four covalent bonds with its host neighbours, but the FIFTH electron has no bond left to join -- it remains only loosely attached to its own impurity atom, requiring very little thermal energy to break free and join the conduction band as a mobile carrier. The impurity atom, having donated this electron, is called a donor, and the resulting material -- with a large population of free electrons contributed by the donors, vastly exceeding the small number of thermally-generated intrinsic carriers -- is called an n-type semiconductor (majority carriers: electrons; minority carriers: the few thermally-generated holes).

p-type semiconductor (trivalent doping). Doping instead with a trace of a TRIVALENT impurity atom (Group 13: boron, aluminium, gallium, indium -- three valence electrons) has the opposite effect: the impurity atom can complete only THREE of the four bonds its lattice site requires, leaving the fourth bond incomplete -- a hole. This incomplete bond readily ACCEPTS an electron from a neighbouring host bond (again with very little thermal energy needed), which fills the impurity's own vacancy but creates a new mobile hole at the neighbour's former bond -- so the impurity atom is called an acceptor, and the resulting material, with a large population of holes contributed indirectly by the acceptors, is called a p-type semiconductor (majority carriers: holes; minority carriers: the few thermally-generated electrons). …