Comparison of low-temperature GaN, SiO ₂, and SiN _x as gate insulators on AlGaNGaN heterostructure field-effect transistors

The performance of AlGaNGaN heterostructure field-effect transistors (HFETs) with either uncapped surfaces or with low-temperature (LT) GaN or Si O2 or Si Nx as gate insulators is reported. The sheet carrier concentrations of AlGaNGaN HFETs with any of these surface insulating layers are similar to each other and in each case about 50% higher than that in an AlGaNGaN 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.