This article reports an InAlAs/InGaAs metal oxide semiconductor metamorphic high electron mobility transistor (MOS-MHEMT) by using ozone water oxidation treatment to form an 8.5 nm thick gate oxide with a superior surface flatness. The proposed MHEMT with (without) ozone treatment has demonstrated a lower gate leakage density of 2 mu A/mm (0.48 mA/mm) at V-gd = -5 V, improved output conductance (g(d)) of 8.5 (33.1) mS/mm, gate-voltage swing of 0.9 (0.45) V, enhanced output power of 18.34 (13.43) dBm, and power-added efficiency of 46.8 (26.3)% at 300 K, with gate dimensions of 1 x 200 mu m(2). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3407624] All rights reserved.