Thin AIN film was deposited as a passivation layer on the surface of post-processed AlGaN/GaN heterojunction field effect transistors (HFETs) by radio-frequency (RF) plasma induced molecular beam epitaxy. The AIN layer was grown at 150 degreesC in a migration enhanced epitaxy (MEE) mode using alternating beams of Al and N. Addition of this surface layer reduced RF channel current slump and RF full channel current at 8 GHz increased from 400 to 800 mA/ mm. The saturated output power density of 0.3 mum x 100 mum HFETs at 8 GHz was doubled from 0.8 to 1.6 W/mm. Other characteristics of HFETs before and after the AIN deposition and materials properties of the low temperature grown AIN were measured. AIN surface layer appears an effective passivation for AlGaN/GaN HFETs. (C) 2003 Elsevier Ltd. All rights reserved.