GaAs MESFET's suitable for operation in the millimeter-wave frequency range have been developed. These devices feature electron-beam-defined sub-half-micrometer gates with MBE grown materials. With an active-layer doping of 6 × 1017/cm3, an extrinsic transconductance of 330 mS/mm was obtained. A 75-µm gate-width device has achieved a gain of 13, 9.5, and 6.5 dB at 35, 44, and 60 GHz, respectively.