The behaviour of n + self-aligned short-channel normally-off GaAs MESFETs with increasing channel doping concentration is investigated by detailed computer simulation. Characteristic data of n + self-aligned technology like n +-gate spacing and projected range of n +-implant are optimized. For the calculation a two-dimensional numerical GaAs device simulation program developed at Dresden University of Technology is used. Increasing channel doping concentration up to 10 18 cm −3 decreases sensitivity of threshold voltage upon gate length by about 25–30%. Furthermore, transconductance and K-value are increased by more than twofold. A transconductance of 610 mS/mm was calculated for a GaAs normally-off MESFET with 0.5 μm gate. This is a remarkably high value for MESFETs.