A general assumption is that electrically active dopant atoms reside on substitutional sites in the lattice. A dopant atom at a substitutional site in an otherwise perfect silicon lattice could diffuse only via direct exchange or ring mechanisms. The activation energy for these mechanisms is very high [Hu73]. This means that dopants occupying a substitutional site in silicon can hardly move without the presence of point defects. These point defects, vacancies and silicon self-interstitials in various charge states, act as diffusion vehicles.