Incorporation of Si ion implantation to GaN metal semiconductor field effect transistor (MESFET) processing has been demonstrated. The channel and source/drain regions formed using Si ion implantation into undoped GaN on sapphire substrate. In comparison with the conventional devices without ion implanted source/drain structures, the ion implanted devices showed excellent device performance. On-state resistance reduces from 210 Ω-mm to 105 Ω-mm. Saturation drain current and maximum transconductance increase from 36 mA/mm to 78 mA/mm and from 3.8 mS/mm to 10 mS/mm, respectively.