The bias dependent series resistance (R-s) and density distribution profile of interface states (N-ss) of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructure have been investigated by admittance measurements in a wide temperature range of 80-400 K. Experimental results show that both the values of capacitance (C) and conductance (G/omega) were found to be strongly temperature and applied bias voltage dependent. The capacitance-voltage (C-V) plots give a peak especially at high temperatures in the depletion region and disappear at low temperatures. When the temperature was increased both the values of C and G/omega reduced. This decrease of capacitance can be understood if the dielectric constant of interfacial layer decreases with increasing temperature. Such temperature dependent behavior of C and G/omega also can be attributed to the thermal restructuring and reordering of the interface states. The downward curvatures in C-V plots have been attributed to the R-s of heterostructure and R-s may cause a large deviation in C-V and G/omega-V data. Therefore, the entire C-V and G/omega-V measurements were corrected for the effect of R-s. Also, the value of R-s increases with increasing temperature. The density distribution profile of N-ss was obtained from high-low frequency capacitance method. In addition, the temperature dependent ac conductivity (sigma(ac)) data obtained between 275 and 400 K show a linear behavior and was fitted to the Arrhenius plot. The E-a values were obtained from the slope of ln sigma vs 1/T plots to be -58.5, -59.7, -60.4 and -60.6 meV for the applied biases of -4, -3, -2 and -1 V, respectively. (C) 2009 Elsevier B.V. All rights reserved.