An N channel metal-oxide-semiconductor field-effect transistor (MOSFET) with a 0.15 mu m gate was fabricated by using focused ion beam lithography for gate level and its electrical characteristics were investigated. The choice of the ion species and the gate structure were designed to avoid contamination and damage due to ion penetration. The MOSFET showed an excellent junction leakage current and subthreshold swing. The impact of the ion irradiation on the thin gate oxide quality was also investigated.