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Abstract Polar crystals of stoichiometric composition at low temperatures are insulators of electricity. If, however, electrons are raised into the normally empty conduction band of energy levels, either through the absorption of light or the thermal energy of surrounding atoms, the crystal can conduct. The purpose of this note is to calculate the mean free path of such electrons, and hence their mobility (velocity of drift in unit field). The results obtained will be compared with experimental material obtained from semi-conductors and from substances which show photoconductivity.
Fröhlich et al. (Fri,) studied this question.