Au/ZnO/n-GaAs Schottky barrier diodes (SBDs) have been examined by the capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements. The frequency dependence characteristics of measurements were obtained under various illumination levels at room temperature. The C and G/omega relation was observed as the decrement in capacitance corresponds to an increment in conductance. The increment of negative capacitance (NC) values by high frequency at forward biases was ascribed to the series resistance, interface states and interfacial layer. Considering the illumination intensity, the NC values were observed to increase with the decreasing illumination while the G/omega values increase with the increasing illumination. This behavior was referred to the increments in the polarization and carriers in the SBDs. The adverse impacts of the voltage dependent resistivity were decreased with increasing illumination levels. Eventually, a strong interaction between the electrical properties of SBDs and the frequency, illumination and applied bias voltage was demonstrated by experimental results.