This thesis reports the development of a model to explain the electrical properties of Si implanted GaAs. The results show that most of the implanted silicon atoms occupy lattice sites and are electrically active. The net carrier concentration is determined by the relative concentration of silicon atoms on gallium and arsenic lattice sites respectively. The activation mechanism is shown to involve the breaking up of complex defects in the form of substitutional silicon with vacancies. The energy required for this process is about 1 to 1.5 eV. A lower value of activation energy (about 0.5 eV) has also been measured and is suggested to be associated with the site switching of silicon from arsenic to gallium sites, when a gallium vacancy diffuses close to a silicon on an arsenic site. This process has diffusion energy of about 2.5 to 3.0 eV. The activation energy obtained from sheet carrier concentration measurements corresponds to a combination of the two activation mechanisms. Which of these mechanisms is observed in an experiment depends on various parameters, such as the implantation conditions, the quality of the encapsulant and the annealing conditions. The model can explain the variations in activation energy (0.5 to 1.5 eV) reported in the literature.