Molecular beam epitaxy deposited MgO shows low interface state densities (2-3 x 10(11) cm(-2) eV(-1)) on GaN and shows excellent long-term stability in MOS structures on GaN. The effects of oxide thickness and gate length on the threshold voltage and breakdown voltage of MgO/GaN MOSFETs were examined using a drift-diffusion model. Gate breakdown voltage is >100 V for gate length >0.5 mum and MgO thickness >600 Angstrom. The threshold voltage scales linearly with oxide thickness and is <2 V for oxide thickness <800 Angstrom and gate lengths <0.6 mum. (C) 2003 Elsevier Science Ltd. All rights reserved.