An AlGaN/GaN hetero field effect transistor (HFET) was operated above 10 A, and the on-state resistance of the unit HFET was lower than that of a Si-based FET at the breakdown voltage of 100 V. A undoped Al0.2Ga0.8N(30 nm)/GaN(2 mum) heterostructure was grown on the sapphire substrate using a gas-source molecular beam epitaxy. The mobility of Al0.2Ga0.8N/GaN heterostructure was about 1200 cm(2)/Vs at room temperature. We investigated the breakdown voltage of undoped GaN layer. The breakdown voltage of undoped GaN layer was over 2000 V (2 MV/cm). Before the formation of electrodes, Si-doped GaN with a carrier concentration of 5x10(19) cm(-3) was selectively grown in the source and drain regions in order to obtain a very low contact resistance. After that, a large-size Al0.2Ga0.8N/GaN HFET was fabricated. The FET structure was formed using a dry-etching technique. The gate width was 20 cm and the gate length was 2 mum. The distance of source and drain was 6 mum. The electrode materials of the source and the drain were Al/Ti/Au and the Schottky electrodes were Pt/Au. Multi-electrode structures were also fabricated using SiO2 for isolating the source, drain and gate electrodes, respectively. The HFET was operated at a current of over 10 A. The on-state resistance of the HFET was about 2 mOmegacm(2). The trans conductance (g(m)) of this HFET was about 120 mS/mm. It was also confirmed that the breakdown voltage of schottky property was over 600 V. Therefore, a high power AlGaN/GaN HFET was thus demonstrated.