A detailed study of the activation kinetics of silicon implanted GaAs has been carried out for implantation energies ranging from 100 keV to 2 MeV, with ion fluences chosen to give silicon concentrations in the useful range of between 10 17 and 10 19 Si cm -3 for all the implant energies studied. Anneal times have been varied over three orders of magnitude to give information relating to both RTA and furnace anneals with the temperatures being varied from 700 to 1150°C. The effect of compositional changes in the dielectric encapsulant has also been studied and shown to be a major variable in the activation mechanism. SIMS analysis has been compared with TRIM simulations to give information on the atomic concentration of the implanted silicon. Following annealing, differential Hall effect measurements have been used to obtain electron concentration and mobility profiles for comparison with the atomic concentration and implant induced disorder profiles. In this paper, the role of implantation damage, vacancy distribution and implant dopant concentration is discussed in conjunction with a model for the activation of silicon implants in GaAs.