We fabricated a 0.5-μm gate MESFET on a bulk semi-insulating 4H-SiC substrate by using ion implantation for the channel layer and contact region. Nitrogen ions were implanted to obtain a 0.25-μm-thick box-shaped profile with a doping density of 3.0×1017/cm3 for the channel region and to obtain a 0.2-μm-thick box-shaped profile with a doping density of 2.0×1020/cm3 for the contact region. Activation annealing is done in argon ambient at 1300 °C for 30 minutes. A 0.5- μm gate MESFET with 100-μm gate width showed a cut-off frequency of 7.5 GHz and a maximum oscillation frequency of 22.2 GHz. And its saturated output power was 25 dBm (3.16 W/mm), power gain was 6.7 dB and PAE was 15.7%.