Tantalum silicide (TaSix ) films with different Si to Ta ratios (x) as Schottky contacts to GaAs have been studied. The films were prepared by coevaporation. The metallurgical properties of the film and the stability of the silicide-GaAs interface were studied by x-ray diffraction, Auger electron spectroscopy, and Rutherford backscattering spectroscopy. The films were annealed at various temperatures (up to 850 °C) for thermal stability evaluation and the Schottky diodes were characterized by I-V and C-V measurements. It was found that the Si to Ta ratio (x) plays an important role in the thermal stability of the Schottky diode. For small x values, there are interactions between Ta and GaAs, probably a compound formation, after high-temperature annealing. For large x values, the degradation mechanism for the Schottky diodes after high-temperature annealing appears to be the out-diffusion of Ga and As from the substrate. The best composition for a thermally stable Schottky barrier is Ta5 Si3, which shows stable Schottky characteristics after annealing with temperatures up to 800 °C.