Enhancement-mode AlGaN/GaN high-electron-mobility transistors (HEMTs) were fabricated on a Si substrate by fluorine plasma treatment without pre-etching. The threshold voltage shifted from %8 to 2.3 V, effectively converting the depletion-mode HEMT to the enhancement-mode HEMT. The leakage current was reduced by two orders of magnitude by CF4 plasma treatment. The device exhibited a superior performance with a maximum drain saturation current of 210mA/mm at VGS = 10V and a peak gain of 44.1mS/mm. A low off-state gate leakage current of 10-6mA/mm, ION/IOFF ratio of approximately 108, and high breakdown voltage of 1480V were obtained.