This work proposes a two-step passivation approach for AlGaN/GaN high-electron-mobility transistors (HEMTs) and demonstrates improved dc, high-frequency and microwave power performance. These improvements depend primarily on the pretreatment of the AlGaN surface provided by the selective dry etching of n(+)-GaN cap layers and the subsequent RTA annealing of ohmic contacts, both of which steps are performed immediately before the first-step passivants are deposited. No additional process step is adopted to prepare the surface for passivation. PECVD-deposited SiN(x) and e-beam-evaporated SiO(x) are selected as the passivants in this work and both effectively suppress trapping effect when used in the two-step passivation approach. (C) 2010 Elsevier Ltd. All rights reserved.