Abstract
Transport measurements in n-type three-dimensional GaAs with net donor concentration below the metal-insulator transition have been performed at very low temperatures 20lTl300 mK and with electric fields up to 25 V/cm. In the Ohmic regime, the mechanism of conduction is variable-range hopping with a Coulomb gap in the density of states at the Fermi level. Under the extreme conditions of high electric field and low temperature, hopping is observed to become activationless. We present evidence that electron correlations are important and that a Coulomb gap is also present in the regime of activation-free phonon-emission-assisted hopping. The relationship between the critical electric field above which transport becomes activationless and temperature is also studied. All results are in agreement with theory.
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