Origin of positive fixed charge at insulator/AlGaN interfaces and its control by AlGaN composition

The key feature for the precise tuning of Vth in GaN-based metal-insulator-semiconductor (MIS) high electron mobility transistors is the control of the positive fixed charge (Qf) at the insulator/III-N interfaces, whose amount is often comparable to the negative surface polarization charge ( Q p o l −). In order to clarify the origin of Qf, we carried out a comprehensive capacitance-voltage (C-V) characterization of SiO2/AlxGa1–xN/GaN and SiN/AlxGa1–xN/GaN structures with Al composition (x) varying from 0.15 to 0.4. For both types of structures, we observed a significant Vth shift in C-V curves towards the positive gate voltage with increasing x. On the contrary, the Schottky gate structures exhibited Vth shift towards the more negative biases. From the numerical simulations of C-V curves using the Poisson's equation supported by the analytical calculations of Vth, we showed that the Vth shift in the examined MIS structures is due to a significant decrease in the positive Qf with rising x. Finally, we exa...