The growth of MgO thin films on p-InP (100) substrates by using electron-beam deposition at a relatively low temperature (similar to 200 degreesC) was performed in order to produce high-quality MgO/p-InP (100) heterointerface and MgO insulator gates with dielectric constants of low magnitude. Atomic force microscopy and X-ray diffraction measurements showed that the MgO films grown on the InP substrates were polycrystalline thin layers with very smooth surfaces. Transmission electron microscopy and selected-area diffraction measurements showed that the grown MgO films were polycrystals with small domains and that the MgO/InP (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, and the interface state densities at the MgO/p-InP interfaces, as determined from the C-V measurements, were approximately 4.5 x 10(11) eV(-1) cm(-2) at an energy of about 0.7 eV below the conduction-band edge. The dielectric constant of the MgO thin films, as determined from the C-V measurements, was as low as 9.67. These results indicate that the MgO layers grown on p-InP (100) substrates at low temperature hold promise for potential electronic devices based on InP substrates.