Monte Carlo simulation of short channel heterostructure field-effect transistors

Self-consistent Monte Carlo simulation was used to study self-aligned, planar-doped AlGaAs-GaAs heterostructure field-effect transistors (HFETs) with gate lengths varying from 0.1 to 1.0 mu m and two different depths of the implanted contacts. The drain output conductance in saturation as well as the threshold voltage shift are found to be approximately inversely proportional to the square of the gate length for gate lengths smaller than 0.5 mu m. The predominant physical mechanism behind these short-channel effects at such gate lengths is the injection of electrons from the contacts into the GaAs buffer region beneath the two-dimensional channel. The critical parameter for the onset of large short-channel effects is the ratio between the source-region-drain-region separation and the contact depth. Hence, an optimum depth of the contacts should be found as a tradeoff between short-channel effects and parasitic series resistances. Simulated current-voltage characteristics exhibit pronounced negative differential resistance at large gate voltages because of real space transfer of channel elections into the AlGaAs layer and subsequent collection by the gate electrode. Simulated on and off transients have similar durations, but a trend toward shorter switch-off times exists.<
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