GaN and related compound semiconductors, as well as SiC and diamond, are wide bandgap semiconductors that also have high melting points, and high electric breakdown fields. These materials therefore have potential for the power electronic devices that can be operated under conditions of high breakdown voltage, high frequency, and high temperature. In Japan, the development of GaN has progressed to fabricate blue light emitting diodes (LED) and laser diodes (LD). In the U.S.A., research on GaN electronic devices has already begun. However, recently in Japan, research on GaN electronic devices for high-frequency applications started with the 673 K operation GaN metal semiconductor field effect transistor (MESFET) presented by the Furukawa Electric Co. In this research, we paid attention to the excellent figures of merit of GaN, which can be operated under conditions of high current and high breakdown voltage, and successfully performed high-current operation (20 A) of an AlGaN/GaN hetero FET for the first time. The minimum on-state resistance (Ron) was 2 mΩcm2 at 100 V. This is the lowest value for a GaN-based FET, and corresponds to one-quarter of that of Si-devices.