The use of established GaAs technology to satisfy the current requirements for electronic equipment capable of reliable operation at elevated temperatures is the focus of this paper. It is found that the conventional GaAs MESFET fails at lower temperatures than an equivalent Si MOSFET, despite its wider band gap. Computer simulation is used to show that this failure is due to gate and substrate leakage. The wide energy band gap advantage of GaAs can be exploited by the introduction of AlGaAs layers above and below the channel. The proposed device offers reliable high performance up to 300°C