Full Band Monte Carlo Simulation of Wurtzite AlGaN/GaN MODFETs

We present full band Monte Carlo simulations of a wurtzite Al_0.15Ga_0.85N/GaN modulation-doped field-effect transistor (MODFET). We found that without inclusion of the piezoelectric effect, the electron concentrations in the channel are much lower than obtained from experimental data. The calculated I_ds-V_ds curves show a strong negative differential resistance, which is a feature observed in experimental devices. Self-heating effects are usually believed to be the main cause of the negative differential resistance. Our simulations do not include self-heating, and this would indicate that at least part of what is observed is also caused by the drift-velocity behavior vs. electric field of the narrow conduction channel. For a 0.2 μm gate MODFET, the simulations yield a maximum trans-conductance G_m ≈ 250 mS/mm with V_G = 1.0 V and V_ds = 5.0 V. When V_G = 0.0 V and V_ds = 8.0 V, we obtain a maximum cutoff frequency f_T = 180 GHz with I_d = 1159 mA/mm.