An ion-implanted, 0.5 micron gate GaAs MESFET designed for K-band (18-26 GHz) applications is described in this paper. Silicon implantation was used to form both the channel and the N+ contact layers. The device has 9.8 dB gain at 18 GHz and its extrapolated fmax is about 80 GHz. At 25 GHz, it achieved 12.1 dBm output power as a tunable oscillator, and 11 dBmoutput power with 16 dB gain as a narrow band reflection type of amplifier.