Diffusion through interstitial spaces: Impurities which have small ionic radii can travel directly from one interstitial site to another one (see Fig.3.37). Especially Group-I and Group-VIII elements are diffusing mainly interstitially and are therefore fast diffusers.

Vacancy Diffusion: Vacancies are holes in the matrix and they are always moving. An impurity can move into the vacancy and diffuse through the material. A substitutional dopant can exchange its position with the neighboring vacancy. Similarly when a dopant at a substitutional site is kicked out by a silicon interstitial atom, the dopant occupies the interstitial position while the original self-interstitial has disappeared by occupying the regular lattice site.

Thus dopants can diffuse within a crystal lattice using point-defects as diffusion vehicle either by the vacancy mechanism or the interstitial mechanism or a combination of both. The dopants are selected based on their properties such as solubility in silicon, dissolution into lattice sites and ionization. The most common p-type dopant boron diffuses mainly via interstitials, where the n-type dopant phosphorus shows at intrinsic concentrations interstitial dominated diffusion and for high concentrations a dual mechanism with a strong vacancy component. Arsenic is also diffusing via interstitials and vacancies, but the interstitial diffusion is at least limited due to the relatively large ionic radii. Large dopants like Antimony will find no stable position between lattice sites, so they can only diffuse via vacancies.