Point defect means missing of the atoms in the crystal, from the lattice site. In a crystal atoms all the ions, atoms, and molecules are perfectly arranged in a three dimensional space. If we will study these crystals minutely then we will observe some defects in these crystals. Each crystal has some defects present in it. The main reason behind these defects is the impurity of the crystal from which the semi conductor is made. It is almost impossible to make a crystal using a material which is 100% pure.an imperfection in a crystal, characterized by one unoccupied latticeposition or one interstitial atom, molecule, or ion .
Vacancy defects are lattice sites which would be occupied in a perfect crystal, but are vacant. If a neighboring atom moves to occupy the vacant site, the vacancy moves in the opposite direction to the site which used to be occupied by the moving atom.