TRANSPORT-PROPERTIES OF A GATED, DOUBLE QUANTUM-WELL HEMT

Hall and Shubnikov-de Haas measurements have been used to determine the depletion behaviour of a gated GaAs/(al,Ga)As double quantum well structure. A Fourier transform analysis of the Shubnikov-de Haas oscillations has enabled the carrier density in each well to be found as a function of gate bias, and good agreement with a theoretical model based on the numerical solution of the coupled Poisson and Schrodinger equations has been found. Despite the absence of a second maximum in the transconductance characteristics, these results show that depletion of the wells is sequential. These data have enabled the low-field Hall measurements to be analysed to give the dependence of mobility on carrier density in each well; in the near-surface well, a dependence characteristic of remote impurity scattering is observed, but the much steeper variation found for the lower well suggests that localization occurs here.