There are two types of impurity :

• Substitutional impurity - is part of the lattice, replaces an atom from the network
  • the atom travels from a lattice site to another lattice site, without traveling through the interstitials. This can be done by
    • Direct exchange - 6 bonds are broken during the exchange

• Vacancy exchange - only 3 bonds are broken during the exchange - therefore requires less activation energy than direct exchange. Thus, the probability of vacancy exchange is higher than direct exchange. Nevertheless, this phenomena relies on the existance of vacancies in the the lattice.

• Interstitial impurity - is placed in the empty spaces of the lattice (not vacancies), they can become substitional in order to play a role in the network
  • the atom travels from a lattice site to another lattice site, by traveling through the interstitials. The diffusion of the atom has three steps:
    • Removal of the impurity by a silicon self-interstial

• Interstitial impurity diffuses through interstitial vacancies:

• Impurities return to lattice by two possible methods:
  • Frank-Turnball method 
  •  Kick-out mechanism
  • Frank-turnball method requires less energy but depends on the existance of the vacancies in the lattice