AlGaN/GaN heterostructure was grown on semi insulating 6H-SiC substrate. The effect of the thickness of the initial AIN buffer layer on the crystalline quality and the stress of the grown GaN layer were investigated. The semi insulating characteristic of the undoped GaN layer, which is very important for obtaining low device leakage current, was analyzed by photoluminescence measurement Modulation doping of Si during the growth of AlGaN barrier layer was also introduced to increase the concentration of the two-dimensional electron gas density and hence to improve the device performance. The fabricated AlGaN/GaN heterostructure field effect transistor with gate length of 0.2 mu m and SiO2 passiviation layer exhibited good small signal characteristics such as current gain cut-off frequency of 47 GHz and the maximum oscillation frequency of 121 GHz. (C) 2014 Elsevier E.V. All rights reserved.