This work proposes a two-step scheme approach for passivating InAs/AlSb high-electron-mobility transistors (HEMTs) and demonstrates increased output currents and transconductances, reduced off-state leakages, improved subthreshold behaviors, suppressed surface trapping effect, and elevated frequency performance in optical gate devices. These enhancements depend primarily on the pre-passivation of as-grown InAs/AlSb device epitaxies ahead of starting device fabrication using typical plasma-enhanced chemical vapor deposition (PECVD) SiO(2) dielectrics. The pre-passivants improve dielectric-epitaxy interface quality and protect the underlying InAs/AlSb epitaxies from chemical attacks by subsequent processing. No additional process step is required to prepare the surface for passivation. The two-step passivation scheme is finally applied to submicron e-beam T-gate devices and its feasibility for high-frequency applications is successfully demonstrated. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3622848] All rights reserved.