These defects arise when foreign atoms are present at the lattice site (in place of host atoms) or at the vacant interstitial sites. In the former case, we get substitutional solid solutions while in the latter case, we get interstitial solid solution. The formation of the former depends upon the electronic structure of the impurity while that of the later on the size of the impurity.
Consequences of metal deficiency defects
Due to the movement of electron, an ion A+ changes to A+2 ions. Thus, the movement of an electron from A+ ion is an apparent of positive hole and the substances are called p-type semiconductor.