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 T o. 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.