Nowadays, many researches of metal-oxide-semiconductor field-effect-transistor (MOSFETs) using wide band-gap semiconductor materials has been studied. Because of further improvement in the electrical characteristics of Si-based power devices, such as metal-oxide-semiconductor field-effect-transistor (MOSFETs) and insulated gate bipolar transistors, has been restricted by the material limit of Si. This limit can be overcome using the wide band-gap semiconductors. The wide band-gap semiconductors, such as GaN and SiC show as superior material properties than Si. These materials have a high critical field, which enables a higher breakdown voltage at the same drift thickness, or a thinner drift layer at an identical breakdown voltage, compared with conventional Si. Wide band-gap materials also have a low intrinsic carrier concentration even at higher temperatures. Therefore the wide band-gap devices can operate at higher voltages, temperatures, and switching speeds more reliable than current Si technology. As a result, a new generation of power devices has developed for switches in power factor correction circuits, DC/DC converters, DC/AC inverters, and switched-mode power supplies in which traditional Si devices show limited operation. At present, GaN and SiC are the more promising wide band-gap semiconductors devices as a result of their outstanding properties. The MOS capacitors using these wide band-gap semiconductors were fabricated and their characteristics including capacitance-voltage (C-V), leakage current-voltage (I-V) curve, and secondary-ion-mass-spectroscopy (SIMS) were measured. The TiN/SiO2/SiC MOS capacitor without post deposition annealing and additional passivation layer achieves excellent electrical property of the stable C-V with low voltage hysteresis less than 1 mV and high breakdown voltage field. The TiN/Al2O3/AlGaN/GaN MOS capacitors were fabricated and annealed sulfur passivation before deposition of Al2O3 gate oxide to surface treatment for improving the interface properties. The C-V characteristics were changed by sulfur annealing, and the leakage current density abruptly decreased at the annealing temperature higher than 400°C.