Significant improvements in electrical properties are achieved from AlN/Al2O3 stack gate Hterminated diamond metal-insulator-semiconductor field-effect transistors (MISFETs) upon improving the structural quality of an AlN insulating layer. The 5-nm-thick Al2O3 layer and 175-nm- thick AlN film are successively deposited by atomic layer deposition and sputter deposition techniques, respectively, on a (100) H-diamond epitaxial layer substrate. The AlN layer exhibits a poly-crystalline structure with the hexagonal wurtzite phase. The crystallite growth proceeds along the c-axis direction and perpendicular to the substrate surface, resulting in a columnar grain structure with an average grain size of around similar to 40 nm. The MIS diode fabricated provides a leak current density as low as similar to 10(-5) A/cm(2) at gate voltage bias in the range of -8 V and +4V. The MISFET fabricated shows normally off enhancement mode transfer characteristic. The drain-source current maximum, threshold voltage, and maximum extrinsic conductance of the FET with 4 mu m gate length are -8.89 mA/mm, -0.22 V, and 6.83 mS/mm, respectively. Published by AIP Publishing.