It is demonstrated that Si ion implantation is useful to fabricate GaN/AlGaN/GaN HEMTs with extremely low gate leakage current and low source resistance without any recess etching process. The source/drain regions were formed using Si ion implantation into undoped GaN/AlGaN/GaN on sapphire substrate. Using ion implantation into source/drain regions with energies of 30 and 80 keV, the performances were significantly improved. On-resistance reduced from 9.9 to 3.5 Ω·mm. Saturation drain current and maximum transconductance increased from 300 to 560 mA/mm and from 75 to 160 mS/mm, respectively.