Surface state (N-ss) density and their relaxation time (tau) of the fabricated Al/ZnO/p-GaAs structure was investigated in frequency range of 1 kHz-1 MHz and in voltage range of +/- 1.2 V. It was found that the variance of capacitance (C) and conductance (G/omega) with frequency and voltage is basically different especially at low and intermediate frequencies in depletion and accumulation regions. Characteristics of Al/ZnO/p-GaAs structure are affected by parameters such as N-ss, dipole and surface polarization in depletion region at low frequencies, while they are affected by parameters such as interfacial ZnO layer and series resistance (R-s) in accumulation region at higher frequencies. C and G have higher values in low frequency region due to effects such as N-ss in metal-semiconductor interface, interfacial ZnO layer and surface dipole polarization. The rise in C and G has been showed up thanks to the ac signal that can be easily monitored N-ss, strong interface and dipole polarizations, interfacial layer especially at weak inversion and depletion regions. Due to special distribution of N-ss at ZnO/p-GaAs interface, parallel conductance (G(p/omega)) - ln(f) plots show a peak under distinct voltage. Thus, the values of N-ss and omega were determined from the peak values of these plots and changed from 3.92 X 10(12) eV(-1) . cm(2) to 4.33 X 10(12) eV(1) . cm(2) and 6.3 X 10(5) s to 6.3X 10(6) s for 0.35 V and 1.1 V, respectively. These values of N-ss and 1.30 nm RMS roughness (from Atomic Force Microscopy (AFM) image) are very suitable for an electronic device.