Comparative investigations on linearly graded In0.45Al0.55As/InxGa1-xAs (x = 0.53 -> 0.63) metamorphic high-electron-mobility transistors (MHEMTs) with/without applying SiNx surface passivation or double-gate-recess techniques have been comprehensively made in this work. The proposed double-gate-recessed, SiNx-passivated MHEMT with gate dimensions of 0.65 X 200 mu m(2) has demonstrated superior extrinsic transconductance (g(m,max)) of 445 mS/mm, gate-drain breakdown voltage of -13.1 V, intrinsic voltage gain of 69, saturated output power of 14.33 dBm (135 mW/mm), and power-added efficiency of 53.8%, due to its enhanced gate-modulation capability, suppressed gate leakages, and relieved kink effects. The proposed device can be promisingly applied to high-gain and high-power millimeter-wave integrated circuit technologies. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3098480] All rights reserved.