A strain effect has been found to explain the anomalous reduction in P diffusivity at surface concentrations greater than ∼4×1020 cm-3. It is shown that at diffusion temperatures, misfit-induced strain causes a reduction in the effective Si band gap up to ∼-130 meV at the solubility limit of P. This band-gap narrowing results in reduced P diffusivity through a relative reduction of P+V= pairs in the surface region. This complex is the dominant species for P transport at high P concentrations. The diffusion of P in the tail region is dominated by V - vacancies liberated during P+V= pair dissociation events. Thus, the tail diffusivity will likewise be reduced by band-gap narrowing in the surface region. These reductions in diffusivity can be by as much as a factor of 6 depending upon temperature and P doping.