The Al2O3 as a gate oxide and passivation was used to study the transport properties of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). Performance of the devices with Al2O3 of different thickness between 4 and 14 nm prepared by metal-organic chemical vapor deposition (MOCVD) and with 4 nm thick Al2O3 prepared by Al sputtering and oxidation was investigated. All MOS-devices yielded higher transconductance than their HFET counterparts, i.e. the transconductance/ capacitance expected proportionality assuming the same carrier velocity was not fulfilled. A different electric field near/below the gate contact due to a reduction of traps is responsible for the carrier velocity enhancement in the channel of the MOSHFET. The trap reduction depends on the oxide used, as follows from the capacitance vs frequency dispersion for devices investigated. It is qualitatively in a good agreement with the different velocity enhancement evaluated, and devices with thinner oxide show higher traps reduction as well as higher transconductance enhancement. It is also shown that obtained conclusions can be applied well on performance of SiO2/AlGaN/GaN MOSHFETs. (c) 2008 Elsevier Ltd. All rights reserved.