Low (1 × 1014 cm-2) and high (1 × 1015 cm-2) dose implants of Mg+ in undoped GaAs have been activated using the enhanced overpressure proximity (EOP) rapid thermal annealing (RTA) technique. Hall measurements have yielded electrical activation efficiencies as high as 86% and 38% for low and high dose implants, respectively. For high dose implants, the outdiffusion of Mg+ from the wafer surface reduces the activation efficiency. A dramatic improvement in the activation for high dose Mg+ (1×1015 cm−-2, 100 keV) implants has been obtained by the co-implantation of As+. Compared with an activation of 18% for an implant of Mg+ only, the co-implantation of As has increased the activation to as much as 61% with a concomitant sheet resistance of 136 Ω/□. The placement of the As+ implant with respect to the position of the Mg+ profile has been found to play a significant role in the activation efficiency. This co-implantation technique in conjunction with the EOP-RTA method has been applied to the formation of thick p+ regions with high surface carrier concentrations, which has important applications in device fabrication for reducing contact resistance.