Comparison of low-temperature GaN, SiO2, and SiNx as gate insulators on AlGaN∕GaN heterostructure field-effect transistors

The performance of AlGaN∕GaN heterostructure field-effect transistors (HFETs) with either uncapped surfaces or with low-temperature (LT) GaN or SiO2 or SiNx as gate insulators is reported. The sheet carrier concentrations of AlGaN∕GaN HFETs with any of these surface insulating layers are similar to each other and in each case about 50% higher than that in an AlGaN∕GaN HFET with a free surface. This result is consistent with the insulator layers providing passivation of surface states that cause the depletion of the channel layer. Due to the closer lattice match with the AlGaN surface layer, the HFET with a LT-GaN layer as the gate insulator shows the best dc and rf device performance, demonstrating that this material is an effective insulator for nitride electronic devices.