AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with recessed-gate geometry have been proposed to implement normally-off operation for power electronics applications. With this structure, two-dimensional electron gas (2DEG) density under the gate region was effectively depleted, thus controlling threshold voltage while maintaining low on-resistance and high current density. We found a strong correlation between recess depth and threshold voltage: the deeper recess etching provided the higher threshold voltage without any significant decrease in other device parameters such as drain current (90 and 76 mA/mm for 25 and 35 nm recess depths, respectively), subthreshold slope (0.36 and 0.42 V decade(-1)), on-off ratio (similar to 10(6)) and field effect mobility (66.7 and 58.3 cm(2) V-1 s(-1)). We have also examined the electrical properties of the HFETs by employing multiple passivation layers. The drain current and breakdown voltage after multiple passivation layers (Al2O3 + SiOx) were improved by approximately 34% (from 135 to 180 mA/mm) and 40% (from 335 to 470 V) compared to that after a single passivation layer (Al2O3 only), which is ascribed to the surface charge controlled effect. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3555069] All rights reserved.