We fabricated an AlGaN/GaN heterojunction field effect transistor (HFET) with a very low on-state resistance. An undoped Al0.2Ga0.8N(30 nm)/GaN(2 mum) heterostructure was grown on a sapphire substrate using gas-source molecular-beam epitaxy. The undoped GaN layer had a high resistivity (above 10 MOmega) and the breakdown field of the undoped layer was about 2 MV/cm. Si-doped GaN with a carrier concentration of 5 x 10(19) cm(-1) was selectively grown in the source and drain regions for obtaining a very low contact resistance. As a result, a very low ohmic below 1 x 10(-7) Omegacm(2) was obtained. After that, an Al0.2Ga0.8N/GaN HFET was fabricated. The gate width was 20 cm and the gate length was 2 mum. The ohmic electrode materials were Al/Ti/Au and the Schottky electrodes were Pt/Au. The distance between the source and the drain was 13 mum. The HFET was operated at a current of over 20 A. A higher switching speed of HFET was obtained. (C) 2002 Elsevier Science Ltd. All rights reserved.