An enhancement/depletion-type GaAs MESFET IC technology has been developed using selective ion implantation into SI GaAs. By reducing the source-gate and the drain-gate spacings down to about 0.4 μm, the unfavourable effect of the surface state1 was effectively lowered, while guaranteeing sufficient breakdown voltages. The minimum propagation delay of 66 ps was achieved in a 15-stage ring oscillator with 1.2 μm-long gate FETs.