The forward and reverse bias I-V. C-V, and G/omega-V characteristics of (Ni/Au) Schottky barrier diodes (SBDs) on the Al0.22Ga0.78N/AlN/GaN high-electron-mobility-transistor (HEMTs) without and with SiNx insulator layer were measured at room temperature in order to investigate the effects of the insulator layer (SiNx) on the main electrical parameters such as the ideality factor (n), zero-bias barrier height (Phi(B0)), series resistance (R-s), interface-state density (N-ss). The energy density distribution profiles of the N-ss were obtained from the forward bias I-V characteristics by taking into account the voltage dependence of the effective barrier height (Phi(e)) and ideality factor (n(v)) of devices. In addition, the N-ss a function of E-c-E-ss was determined from the low-high frequency capacitance methods. It was found that the values of N-ss and R-s in SBD HEMTs decreases with increasing insulator layer thickness. (C) 2010 Elsevier Ltd. All rights reserved.