The profile of the interface state densities (N-ss) and series resistances (R-s) effect on capacitance-voltage (C-V) and conductance-voltage (G/omega-V) of (Ni/Au)/AlxGa1-xN/AlN/GaN heterostructures as a function of the temperature have been investigated at 1 MHz. The admittance method allows us to obtain the parameters characterizing the metal/semiconductor interface phenomena as well as the bulk phenomena. The method revealed that the density of interface states decreases with increasing temperature. Such a behavior of N-ss can be attributed to reordering and restructure of surface charges. The value of series R-s decreases with decreasing temperature. This behavior of R-s is in obvious disagreement with that reported in the literature. It is found that the N-ss and R-s of the structure are important parameters that strongly influence the electrical parameters of (Ni/Au)/AlxGa1-xN/AlN/GaN(x = 0.22) heterostructures. In addition, in the forward bias region a negative contribution to the capacitance C has been observed, that decreases with the increasing temperature. Copyright (C) 2010 John Wiley & Sons, Ltd.