MgO thin films grown on p-GaAs (100) substrates by using electron-beam deposition at a relatively low temperature (similar to 250 degreesC) was performed in order to produce high-quality MgO/p-GaAs (100) heterointerfaces and MgO good insulator gates. Atomic force microscopy and X-ray diffraction measurements showed that the MgO films grown on the GaAs (100) substrates were (110) oriented epitaxial layers with smooth surfaces. Transmission electron microscopy measurements showed that the MgO/GaAs (100) heterointerface had no significant interdiffusion problem. Room-temperature current-voltage and capacitance-voltage (C-V) measurements clearly revealed a metal-insulator-semiconductor behavior for the diodes with MgO insulator gates. The dielectric constant and the interface state density at the MgO/p-GaAs interface were 9.06 and 1.9 X 10(11) eV(-1) cm(-2) at an energy of about 0.7 eV below the conduction-band edge, respectively. These results indicate that the MgO epitaxial films grown on p-GaAs (100) substrates at low temperature hold promise for potential applications in high-frequency electronic devices.