12 - Impurity Band Conduction in Germanium and Silicon: The Electrical Properties of Germanium Semiconductors at Low Temperatures

Abstract

Low-temperature anomalies in the Hall effect—a steep maximum—and a change of the slope of the log resistivity versus 1/T curve in germanium semiconductors, which were first observed by Hung and Gliessman, have been re-investigated using single crystals of N- and P-type germanium of variouscarrier concentrations. By using cross-shaped samples, the influence of contacts was investigated. To estimate the effect of surface layers, varioussurface treatments have been tried and the ratio of surface area to bulkvolume has been changed. It was found that the effects cannot be explained bysurface conduction. To exclude electrical field effects at low temperatures(to about 1·50°K), fields as low as 3 mV/cm, well below breakdown field, have been used. The Hall effect was measured with varying magnetic fields ranging from 200 to 4300 gauss. In the same temperature region where Hall effect and resistivity become anomalous, the magneto-resistive ratio also shows drastic changes. In the temperature range before the Hall maximum is reached the magneto-resistive ratio changes only slowly with temperature, but starts to decrease sharply at the temperature where the Hall effect approaches the maximum and where the resistivity curve changes slope. The discussion of the results indicates that the observations can be described by a model which assumes conduction in two bands, the regular conduction band (valence band in the case of P-type material) and a band with a very small mobility. The sharp decrease of this mobility with decreasing impurity content suggests thatconduction in an impurity band is a plausible explanation of these phenomena