AlGaN/GaN wide-bandgap high electron mobility transistors (HEMTs) have demonstrated impressive power capabilities in the radio frequency range. In order to fully develop the potential of the device, an accurate simulation model is needed. We employ a Monte Carlo technique to investigate stationary electron transport in GaN. We use the simulation data as a basis for the development of analytical models for the numerical simulation of GaN-based electron devices. In particular, we analyze AlGaN/GaN HEMTs of different gate lengths down to 150nm using the two-dimensional device simulator Minimos-NT. We study the penetration depth of the drain/source metal contacts which may build an alloy with the AlGaN supply layer. In order to properly describe the two-dimensional electron gas in the channel, the value of the positive polarization charge density at the AlGaN/GaN interface is assessed. Since the longitudinal electric field in the channel reaches peak values of above 500 kV/cm, a hydrodynamic approach is used to properly model the electron transport and energy relaxation.
We further assess the impact of thermionic emission and field emission (tunneling) effects which critically determine the current transport across the heterojunctions. A significant improvement of the device performance has been achieved by adopting the field plate technique. With its origins in the context of high-voltage p-n junctions, the main functions of the field plate are to reshape the electric field distribution in the channel and to reduce the peak value on the drain side of the gate edge. Using two-dimensional device simulations, we optimize the electric field distribution in the channel by varying the geometry of the geometry of the field plate.