We report on the development of an evaporated diffusion barrier for incorporation into the metallizations currently used in GaAs MMICs. Results indicated that TiN films formed by reactive ion beam assisted evaporation - the simultaneous evaporation of metal atoms and bombardment with reactive ions - were as stable as TiN films formed by reactive sputter deposition and far superior to films formed by reactive evaporation. In addition the formation of these films is compatible with photoresist lift-off techniques. Chemical and electrical analysis has shown that these films are effective barriers against Au, Ga and As diffusion under extensive (up to 1500 hrs) elevated temperature storage (280°C). TiN-Au gate electrodes whose electrical characteristics are comparable to conventional Ti-Pt-Au gates have also been fabricated.