Oxygen ion implantation has been employed, for the first time, to form the high-resistance layer in the surface of GaAs MESFETs. Comparable DC and RF performance were achieved for MESFETs with and without oxygen ion implantation; however, MESFETs with the extra surface oxygen ion implantation demonstrated good breakdown performance, less frequency dispersion of drain current, and good time dependence of drain current at 150 degreesC in air. This enhancement is a result of the lower surface trapping effects resulting form a high-resistance layer by oxygen ion implantation. This approach relaxes the surface passivation problem in a standard GaAs MESFET. (C) 2002 Elsevier Science Ltd. All rights reserved.