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Optimisation of selectively doped (AlGa)As/GaAs heterostructures for TEGFET applications at room temperature is reported. The introduction of a thick (>0.1 Â¿m) highly doped GaAs top layer considerably reduces parasitic resistances and improves device performance. Best results for 1.4 Â¿m gate length at room temperature are: transconductance 220 mS/mm; source resistance 0.2Â¿0.4 mm; contact resistance less than 0.1 Â¿ mm and cutoff frequency 25 GHz. Results from comparable structures without a thick top layer are inferior by about 25%.