On the threshold voltage of normally-OFF AlGaN/GaN heterostructure field effect transistors (HFETs) with p-(Al)GaN gate
A physics-based analytical method has been developed to correctly estimate the threshold voltage of normally-OFF AlGaN/GaN heterostructure field effect transistors (HFETs) with p-(Al)GaN gate. This analysis considers the effect of all polarization charges at different interfaces/surface, the layer structure, doping concentration of deep acceptors in p-(Al)GaN cap layer, incomplete ionization of deep Mg acceptors, out-diffusion of Mg to AlGaN barrier layer as well as the effect of compensated buffer/back barrier. Threshold voltage extracted from this analytical model is rigorously validated with experimental results. This analysis also provides a physical insight of charge distribution, gate voltage division in the heterostructure and the effect of device parameters on the threshold voltage. Besides, this model can also be extended to estimate the threshold voltage of normally-OFF MIS-HFETs with p-GaN gate.