The stability of Schottky barrier gates in GaAs MESFETS has been investigated to allow device processing at elevated temperatures. The incorporation of a Pt diffusion barrier into the TiAu structure gives stability of electrical characteristics up to 350°C and a Ti-W-Au contact is stable up to 400°C. To achieve higher temperature stability for use in a self aligned gate (SAG) technology, a more stable Schottky barrier material such as a TiW (30 at % Ti) alloy has been used. This has been found to be unreliable above 750°C and it has been found necessary to use a tungsten silicide contact W1.6Si (37 at % Si) to achieve reproducible Schottky contacts at the temperature used for ion implantation activation (800°C). SAGFETs have been fabricated using W1.6Si and initial results indicate good yields (91%), highgm(190 mS/mm) and good uniformity (standard deviation = 6.8% for 293 devices).