A SIMPLE-MODEL FOR THE CHARACTERISTICS OF GAAS ALGAAS MODULATION-DOPED DEVICES/

We have derived a simple model based on electrostatics for predicting the density and threshold voltages of the two-dimensional electron gas in a GaAs-AlGaAs heterostructure, and tested it against measurements of the capacitance-voltage characteristics. The model correctly accoun ts for the large decrease in magnitude of the threshold voltage on coo ling from room to liquid helium temperatures, caused by the freezing o f the DX centres below 150 K. It also shows that the concentration of electrons is almost independent of the concentration of donors, due to the layer of frozen DX centres. We have confirmed the model by perfor ming an experiment in which the sample is cooled under a large negativ e gate bias, which keeps the DX centres empty. The threshold voltage a t low temperatures now retains its high-temperature value. Subsequent annealing causes the threshold voltage to move towards its initial low -temperature value as the DX centres refill, before returning to the h igh-temperature value again. The measured values of the capacitance ar e also explained by the model.