A high gm, 375 mS/mm (Vth= -0.09 V), has been achieved from a 0.3 µm long gate GaAs MESFET with a very small short channel effect by employing an MBE grown channel layer. The maximum K - value obtained was 410 mS/Vmm, which is the highest ever reported for GaAs MESFETs. A unique technology, combining sidewall - assisted self - alignment technology (SWAT) and refractory metal gate n+selective ion - implantation technology, was successfully applied to the fabrication of a GaAs MESFET with MBE grown channel layer in this work, resulting in a very low source series resistance of 0.3 Ωmm. FET characteristic dependences on gate length were also compared for FETs with different doping concentrations. The highly doped channel turned out to be effective to reduce the short channel effects and to improve the FET load drivability.