Техническое чтение для энергетиков. Бухарова Г.П. - 48 стр.

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50
coulombic forces, the hole is attracted to the indium. If it moves away, the fourth
covalent bond to the indium is completed, and the indium is left with a net negative
charge. Group III elements are examples of acceptors, so called because they can
"accept" electrons, thereby Introducing holes into the valence band.
The examples chosen to illustrate donors and acceptors are particularly simple,
but they are not the only kind of donor and acceptor. They both belong to a general
class which run be called "impurity centers", as they arise from the introduction of a
chemical impurity into the lattice, and the semiconductors arising from this kind of
imperfection can be called "impurity semiconductors". Certain elements from other
groups of the periodic table may also act as donors and acceptors in substitutional
positions in germanium and silicon.
Certain impurities which enter the lattice in interstitial positions, may also be
donors in semiconductors. Lithium in silicon and germanium is an example. The
neutral lithium atom has a single valence electron, and is known to occupy (normally)
an interstitial site. The odd electron is easily removed, leaving an interstitial positive
lithium ion. The electrical consequences of an interstitial donor, like lithium, are
entirely similar to those of substitutional donors like arsenic. One might ask whether
interstitial acceptors also exist. No such case has been established, and possibly this is
due to the difficulty of fitting a large negative ion into an interstitial site.
The same general scheme holds for donors and acceptors in compound
semiconductors. In Group III –Group V compounds, such as GaAs, one expects, and
finds, that elements Of Group VI, substitution ally replacing arsenic, act as donors,
while elements of Group II, if they occupy gallium sites, act as acceptors. Similar
considerations hold in other compound semiconductors, such as the II-VI
compounds.
ENERGY LEVELS AND ACCEPTORS
A way of introducing donors and acceptors into semiconductors arises from
nonstoichiometry in compounds. Several possible ways this might happen can be
foreseen. The nonstoichiometry can arise either by virtue of vacant lattice sites for
one component of the compound, or because of an excess of one component located
in interstitial sites. Anion or cation excesses or deficiencies might be involved, and
we might be concerned with either donors or acceptors.
To illustrate how nonstoichiometry leads to such effects, we consider only a
single example here. A donor center can result from the trapping of one or more
electrons in an anion vacancy. The classic examples of centers of this type are the F-
rmters in alkali halides, although these materials are not usually considered
semiconductors. The same kind of center is believed to result from nonstoichiometry
in CdS. t ihow in Fig. 3 a crystal of CdS. We show in Fig a crystal of CdS, in which a
few anion lattice sites are vacant, corresponding to a stoichiometric excess of
cadmium. In order to maintain charge neutrality in the crystal, two electrons must be
supplied for every ion removed. In the vicinity of the vacancy, there is a net positive

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