The influence of O-2 plasma treatment on the mesa-isolated region of AlGaN/GaN heterojunction field-effect transistors (HFETs) was studied. The etched surface of the undoped GaN layer was exposed to O-2 plasma generated by a plasma-enhanced chemical vapor deposition system. The current-voltage characteristics indicated that the current of the mesa-isolated region was strongly dependent on the treatment temperature. Treatment with O-2 plasma at 300 degrees C and 250W for 15 min was confirmed to be the optimal condition, under which isolation current was reduced by four orders of magnitude and photovoltaic response was suppressed. The photoluminescence spectrum showed a decrease in the density of defects related to the yellow luminescence band and the occurrence of defects related to the blue luminescence band. X-ray photoelectron spectroscopy results showed the formation of Ga2O3 and a possible defect of substitutional oxygen on the nitrogen site O-N. AlGaN/GaN HFETs with an on/off drain current ratio of 1.73 x 10(7) were obtained, and the breakdown voltage of the mesa-isolated region increased from 171.5 to 467.2 V. (C) 2015 Elsevier B.V. All rights reserved.