The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-insulator-semiconductor (MIS) GaAs Schottky diodes are investigated and compared with metal-semiconductor (MS) diodes. The MIS diode showed nonideal behavior of I-V characteristics with an ideality factor of 1.17 and a barrier height of 0.97 eV. The energy distribution of interface states density was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height, though it is small. The reduction in the saturation current in the MIS case is caused by a thin oxide layer and is due to the combination of increased barrier height and a decrease in the Richardson constant. The carrier concentration anomaly observed between the MIS and MS diodes measured from reverse bias C-V measurements is explained via oxide (beta-Ga2O3) traps due to the Ga-vacancy by deep level transient spectroscopy (DLTS) measurement.