Abstract
We report low-temperature electrical resistivity and magnetoresistivity measurements of doped semiconductors below the critical concentration of the metal-insulator transition. The series of samples were chemically doped (CdTe and Si) or were obtained by the neutron transmutation doping technique (GaAs and Ge). The resistivity was measured in the temperature range T = 0.03-4.2 K. All samples show, at the lowest temperatures, variable-range hopping resistivity p(T) = p o exp [(T o/T) s ] with s = ½. Comparison of the experimentally determined T o with the heoretical single-electron value of Efros and Shklovskii, T o = T ESO = 2.86e 2/k B a B k O, shows that multiple-electron transitions are dominant at all compensations except K ≥ 0.7, where a large scale potential relief exists. In this case the single-electron approach seems to be a good approximation.