Conductivity ofn-type GaAs near the Mott transition

Low-T transport data on n-type GaAs in several regimes from insulating to metallic is presented. On the insulating side of the Mott transition, it is shown that the power s in the variable-range-hopping law, ${\sigma }_v$=${\sigma }_0$exp[-($T_0$/T${)}^s$], which is found to be (1/4 for pure enough samples, can reach (1/2 for a narrow range of higher impurity concentrations. A small metal-like contribution to the conductivity can explain the observed shift in s and is supported by Hall-mobility data. Consequently, the corresponding behavior can hardly be interpreted as due to a Coulomb gap in the density of impurity states. Evidence is presented for weak localization and then normal metallic behavior for higher impurity concentrations.