Dc and microwave models for Al(x)Ga(1-x)As/GaAs high electron mobility transistors

Models are developed for the dc I-V curves and microwave small-signal parameters of the Al(x)Ga(1-x)As/GaAs heterojunction field-effect transistor, called the high electron mobility transistor (HEMT). An analytic velocity versus field model is used, along with the exact variation with density of the GaAs two-dimensional electron gas Fermi level. A numerical integration is used to obtain the drain voltage for a given gate voltage and source-drain current. The resulting I-V curves are in excellent agreement with the experimental data from four different groups. This model is also used to calculate the transconductance and gate capacitance, and a model is developed for the source resistance. These are used to calculate the maximum frequency of oscillation for a range of values of gate length, of AlGaAs alloy composition and doping, and of the thickness of the undoped AlGaAs spacer layer. The results are compared with measured data for HEMT's as well as for a similar GaAs FET with 0.35-micron gate length.