The effects of Sc2O3 passivation on deep level spectra observed in current and capacitance deep level transient spectroscopy (DLTS) measurements performed on AlGaN/GaN transistor structures were studied. The most prominent traps were surface electron traps with apparent activation energies of 1 and 0.9 eV. Both types of traps can be strongly suppressed by Sc2O3 passivation which correlates with improved microwave performance of transistors and decreased leakage current of the Schottky diodes gates. The results are interpreted using a model of formation of a virtual gate at the surface of the transistor structure upon capture of electrons by the surface traps. (C) 2004 The Electrochemical Society.