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GaAs MESFET's with highly doped channels up to cm-3and with both micrometer and submicrometer gates were fabricated and evaluated. FET's with 1.2-µm gates show an extrinsic transconductance of more than 250 mS/mm, cutoff frequencies around 20 GHz, and a noise figure of 2 dB at 8 GHz with 9-dB associated gain. Breakdown voltage is higher than 6 V. FET's with 1.2- and 0.4-µm gates were simultaneously fabricated on the same wafer to investigate short-channel effects. The short-channel devices show a good saturation behavior and no shift in the threshold voltage compared to the long-channel devices thus demonstrating a pronounced alleviation of short-channel effects as experienced for cm-3doping levels. The influence of doping concentration on the performance of devices with micrometer and submicrometer gates upon doping concentration is investigated by detailed computer simulations. Good agreement between theoretical and experimental results is obtained. From these results improved technological approaches are pointed out.