We have found that co-implantation of phosphorus (P) with Si into SI-GaAs caq improve the quality of ion implanted layer. Hall mobilities of 4600–4700 cm2/Vs and activation efficiencies of 75–85% are obtained. These results are better than those obtained from the samples without P co-implantation. Schottky-barrier technique measuerments show a better mobility profile for the samples with P co-implantation. DLTS measurements show that the number of deep levels and their concentrations decrease. We think that P atoms occupy As vacancies and enhance the activation efficiency and their uniformity. The damage produced under the Si-implanted layer by P co-implantation increases the average Hall mobility.