The effect of UV-ozone pretreatment of AlGaN/GaN high electron mobility transistor (HEMT) surfaces prior to SiNx passivation is reported. Without pretreatment, breakdown voltages dropped from 60 V (the test maximum) to similar to 5 V, and interdevice isolation current increased from 10(-9) to 10(-6) A following plasma-enhanced chemical vapor deposition SiNx passivation. With pretreatment, breakdown voltages dropped from 60 to similar to58 V, and interdevice isolation current increased from 10(-9) to similar to10(-8) A following SiNx passivation. Transmission electron microscopy and X-ray photoelectron spectroscopy measurements showed the formation of a 15-25 Angstrom thick GaO layer as a result of the UV-Ozone treatment. HEMT device results showed that UV-ozone pretreatment does not degrade the ability of the SiNx to provide effective surface passivation and reduce current collapse. (C) 2004 The Electrochemical Society.