Hopping in a low-mobility GaAs-AlGaAs heterojunction in the limit of low electronic concentrations

A gated low-mobility GaAs-Al0.3Ga0.7As delta-doped heterojunction of large area is used to demonstrate a transition from diffusive to hopping transport as the electron concentration in the two-dimensional conducting layer is reduced. At helium temperatures (1.3<T<4.2 K), the hopping conductivity is well described by the variable-range-hopping law sigma approximately sigma 0 exp-(T0/T)1/2 expected when electron-electron interactions are strong. At very low temperatures (T approximately=100 mK), conductance fluctuations are seen, indicating that under these conditions size effects are important and a description of transport by a macroscopic model of conduction is inappropriate.