The authors investigate 2 mu m gate-length InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS HEMTs) with 12 nm thick Al2O3 gate insulation. Compared to the Schottky barrier (SB) HEMT with similar design, the MOS HEMT exhibits a gate leakage reduction by six to ten orders of magnitude. A maximal drain current density (I-DS=0.9 A/mm) and an extrinsic transconductance (g(me)=115 mS/mm) of the MOS HEMT also show improvements despite the threshold voltage shift. An analytical modeling shows that a higher mobility of electrons in the channel of the MOS HEMT and consequently a higher number of electrons attaining the velocity saturation may explain the observed increase in g(me) after the gate insulation. (C) 2007 American Institute of Physics.