The impact of high temperature rapid thermal annealing (RTA) on the mode of operation of AlGaN/GaN modulation doped field effect transistors (MODFETs) is reported. It is observed that annealing at high temperatures is capable of turning the normally depletion-mode (D-mode) characteristics of an AlGaN/GaN MODFET, towards that of an enhancement-mode (E-mode). This change is shown to be partly reversible through UV illumination. These results support the arguments on the extensive role of deep surface states on the operation of AlGaN/GaN MODFETs. According to this variation of characteristics, fabrication and characterization of close to E-mode AlGaN/GaN MODFETs are reported, using MBE grown material on sapphire. The devices demonstrate maximum extrinsic gate transconductance, of 180 mS/mm. Unity current gain cutoff frequency (f(T)) of 5 GHz and maximum oscillation frequency (f(max)) Of 14 GHz were measured. (c) 2005 Elsevier Ltd. All rights reserved.