Equilibria Involving Foreign Atoms

Foreign atoms or impurities may enter a crystal lattice either interstitially or substitutionally. Consequently, they also may be considered as lattice defects such that their interactions with native defects and normal lattice atoms can be treated by the mass action law. 

Impurities will generally affect the electrical properties of crystals by introducing ionizable centers. Interactions between foreign atoms 

Interstitial defects in covalent elemental crystals will usually act as electron donors due to simple ionization for example, copper in germanium  

The copper interstitial also may become doubly ionized  

Substitutional impurities may act as donors or acceptors depending upon the number of valence electrons on the impurity atom as compared to the host atoms. For example, phosphorus in silicon will act as a donor  

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