High-kappa insulating materials (HfO2, HfO2/Al2O3, HfAlOx, and HfSiOx) were deposited by atomic layer deposition (ALD) on AlGaN/GaN to form Metal-Insulator-Semiconductor Heterostructure Field Effect Transistors (MISHFETs) and were electrically and structurally characterized. The objective of this study is to characterize the interface quality and correlate the results with electrical phenomena for each insulating material. Although there are many studies using HfO2 and Al2O3 on AlGaN, there is limited experimental data using ternary compounds such as HfAlOx or HfSiOx, compared to their binary counterparts. In this work, interface trap density, D-it, was extracted by the conductance method using on-chip metal-insulator-semiconductor heterostructure capacitors (MISHCAPs). HfO2 was measured to have the lowest trap density at low energies on the order of 10(12) cm(-2) eV(-1) and quickly reduced about one order of magnitude less than the others at higher trap energies. HfO2/Al2O3, HfAlOx, and HfSiOx all had similar trap densities on the order of 10(12) cm(-2) eV(-1). Ultra-low gate leakage levels were achieved, especially for HfAlOx on the orders of 10(-12) A/mm. Our studies indicate that HfAlOx provides the best electrical characteristics such as lowest gate leakage current, largest channel carrier density and resistance to self-heating effects without the vulnerability to low crystallization temperatures. (C) 2014 Elsevier Ltd. All rights reserved.