Miniband transport in GaAs–AlAs superlattices with narrow band widths is investigated by electrical time-of-flight experiments as a function of temperature. Negative differential velocity is observed in all cases. The low-field drift mobility is inversely proportional to the temperature above 40 K, indicating miniband transport in the nondegenerate case with a temperature-independent scattering time. Below 40 K, the temperature dependence shows the signature of hopping transport. The occurrence of these two transport regimes can be taken as evidence for the existence of a mobility gap in these superlattices.

• Received 29 October 1990

DOI:https://doi.org/10.1103/PhysRevB.43.12094