Page 258 - Materials Chemistry, Second Edition

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4.1. Properties and Types of Semiconductors

Figure 4.7. Illustration of creation/migration of electrons and holes created through Si–Si bond
thermolysis. Shown are (a) the release of an electron and concomitant formation of a hole and (b) the
migration of an electron from a nearby bond to fill the vacancy.

Conduction Band

Energy hn or D

Valence Band

Figure 4.8. Schematic of recombination of electron-hole pairs generating either a photon of energy or heat.

throughout the lattice. However, the vacancies (i.e., holes) left behind are also free to
move – in the opposite direction as electrons. One may consider these holes as
positively charged species formed from loss of an electron. Thus, electrons and holes
represent the two types of carriers that correspond to electrical conductivity in
semiconductors.
Since an electron that has been promoted to the conduction band will have a
greater energy than those left in the valence band, there is a possibility for the
electron to lose this excess energy. The spontaneous return of electrons in the
conduction band to the valance band is known as recombination, and is usually
accompanied by light emission and heat (Figure 4.8). This phenomenon happens all
the time for excited-state molecules. For instance, consider what happens when one