Carrier concentrations at a level of ≳1×1019 cm -3 were achieved when Si-capped GaAs underwent rapid thermal oxidation (RTO) in Ar+0.1% O2 ambient at 850-1000°C for 10-60 s followed by rapid thermal annealing (RTA) in Ar ambient at 850-950°C. Carrier concentrations in the RTO only samples were in the range of 2-5×1018 cm-3. Kinetic data on the diffusion of Si under RTO and RTO+RTA conditions are presented. The enhancement in the electrical activation of the diffused Si during RTA appears to be partly due to its local atomic rearrangement and partly due to redistribution in the GaAs. Ohmic contacts to the doped layer were made using Au-Ge-Ni alloy and contact resistances of ≲0.1 Ω mm were obtained.