Si ion-implanted GaN/AlGaN/GaN HEMTs with extremely low gate leakage current and low source resistance without any recess etching process are demonstrated. 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 energy of 80 keV, the performances were significantly improved. On-resistance reduced from 26.2 to 4.3 Ω·mm. Saturation drain current and maximum transconductance increased from 284 to 723 mA/mm and from 48 to 147 mS/mm.