(Ni/Au)AlxGa1-xN/AlN/GaN(x = 0.22) heterostructures with and without a passivation layer of the SiNx were fabricated in order to see the effect of the insulator layer on the main electrical parameters such as zero-bias barrier height (BH) (Phi(B0)), ideality factor (n), series resistance (R-s) of the structure, and the interface state density (N-ss). Some of these parameters were determined from both I-V and admittance (C-V and G/omega-V) measurements at room temperature and at 1 MHz and were compared. The experimental results show that the value of N-ss in a Schottky contact without passivation is nearly 1 order of magnitude larger than that in a Schottky contact with SiNx passivation layers. Also, the values of R-s increase with the increasing thickness of the passivation layer. In the forward bias region, the negative values of capacitance are an attractive result of this study. This negative capacitance disappears in presence of the passivation layer. Copyright (C) 2010 John Wiley & Sons, Ltd.