In this work, we studied the thermal stability of a plasma-PH3 passivated HfAlO/InGa0.53As0.47 gate stack for high speed metal-oxide-semiconductor field effect transistor (MOSFET) application, and results show excellent thermal stability up to 800 degrees C with negligible changes in the equivalent oxide thickness, interface trap densities (D-it), and subthreshold slope. An increase in leakage current (1.10 x 10(-4) A/cm(2) at V-g = 1.5 V) after 800 degrees C anneal can be attributed to the localized thinning of the gate dielectric and the rough interface caused by the out-diffusion of Ga/As, as observed by transmission electron microscopy and energy dispersion X-ray analysis. D-it measurement by the charge-trapping method showed suppressed D-it at the upper half of the bandgap for plasma-PH3 passivated devices, which is favorable for n-channel MOSFET operation. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3465300] All rights reserved.