POSITIVE AND NEGATIVE MAGNETORESISTANCE IN THE VARIABLE-RANGE-HOPPING REGIME OF DOPED SEMICONDUCTORS

Experimental results are reported on high-field positive and negative magnetoresistance in semiconducting materials in which variable-range hopping occurs at low temperatures. In the case of positive magnetoresistance, both the Mott (T-1/4) and the Shklovskii-Efros (T-1/2) regimes, are observed. We discuss the transition between these two regimes, the shape of the Coulomb gap, and the scaling behaviour of the parameter T0. At low magnetic fields and for different semiconducting materials, we have observed a negative magnetoresistance associated with variable-range-hopping conduction. Experimental data are tentatively compared with available models for orbital magnetoresistance mechanisms, based on quantum interference between tunnelling paths in the insulating regime.