Two-Dimensional Simulation of the High Electron Mobility Transistor

131 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.The High Electron Mobility Transistor (HEMT), a recently developed electronic device which takes advantage of the excellent conduction properties of modulation-doped GaAa/AlGaAs semiconductors, has been shown to perform well for both high-speed digital and analog applications. Switching speeds on the order of 10 ps and transconductances in excess of 400 mS/mm have been demonstrated.In this thesis we develop a theoretical model with which we examine the operation of such a device. The electronic transport in GaAs, because of polar-optical scattering and low subsidiary valley minima, is not easily modeled, particularly for small geometry devices where electric fields are large and electronic heating is significant. Our model includes such effects by means of the first four moments of the Boltzmann equation and treats both concentration and average energy as dependent variables. The parameters of these equations are taken as functions of average energy which are determined from steady-state Monte Carlo simulations and experimental results. Our model also includes conduction outside the modulation-doped quantum well, a condition that is favored in a hot electronic system.We calculate results with a two-dimensional numerical technique for both steady-state and transient operation. For a 3 micron device at 77K we calculate a transconductance of 450 mS/mm, a current-switching speed of 6 ps, and a capacitive charging speed of 4 ps per fan-out gate which corresponds to the measured performance of other workers. We also see that electronic heating, velocity overshoot, and conduction outside the quantum well are significant near the pinch-off point. We conclude that the advantage of the excellent conduction in the quantum well is not in a high saturation velocity at pinch-off but rather in a low access resistance. In addition, we observe that when the device is in current saturation, the various quantities at the pinch-off point are not independent of the drain potential. We conclude that current saturation in the HEMT cannot be explained in terms of a velocity saturation mechanism at the drain edge of the gate, but is better explained as a current injection into the space-charge region of a pinch-off point.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD