Ion-implanted GaAs MESFETs with and without extra oxygen implantation have been fabricated, studied and compared. The extra oxygen implantation was employed after the channel implantation to form a high-resistance layer near the surface of GaAs MESFETs and to replace the passivation process. Two different oxygen implantation and annealing conditions were used to study the effect of implanted oxygen on device performance. The device with oxygen implantation and annealing at the same time as channel implantation demonstrated improved transconductance and breakdown characteristics, less frequency dispersion of drain current and good time dependence of drain current at 150 • C in air without sacrificing rf performance.