Contacts consisting of tungsten, W0.795i0.21 , W0.675i0.33 and W0.578i0.43 have been produced by sputtering on GaAs(001) and annealed isochronally for 20 min in an arsenic overpressure at temperatures ranging from 500 to 950 Β°C. Subsequently, solid state reaction products, microstructure and phase morphology were characterized by cross-section transmission electron microscopy and glancing-angle X-ray diffraction, whereas carrier concentration profiles, Schottky barrier heights and corresponding ideality factors were measured by capacitance-voltage and current-voltage techniques. Contacts consisting of W/GaAs are characterized by an equilibrium microstructure (fl-W and/or a-W/GaAs) under all conditions, but corresponding electrical properties degrade substantially during annealing at 750 Β°C and above. In contrast, W0.79Si0.21/Gams and W0.67Si0.33/Gams contacts are characterized by an amorphous structure after annealing up to 750 Β°C, but revert to an equilibrium microstructure (a-W and WsSi 3 on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade less severely during annealing at 750 Β°C and above. Finally, W0.57Si0.a3/Gams contacts are characterized by an amorphous structure after annealing up to 500 Β°C, but revert to an equilibrium microstructure (WsSi 3 and WSi 2 on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade during annealing at 500 Β°C and above. In general, degradation of electrical properties is attributed to a combination of interdiffusion across and solid state reactions at the WSix-GaAs interface. A previously unreported cubic phase is also observed to form at the W0.57Si0.a3-GaAs interface during annealing at 750 Β°C and above.