We have developed a theory of impurity diffusion in silicon under equilibrium and nonequilibrium concentrations of point defects. The results of first-principles calculations of several key quantities are combined with this theory and compared to experimental data. We find that vacancies and self-interstitials mediate the equilibrium diffusion B, P, and As with comparable activation energies, but interstitials are domoinant. Sb diffusion, on the other hand, is mediated primarily by vacancies. We also find that the direct-exchange mechanism plays only a minor role for all dopants studied. © 1989 The American Physical Society.