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.
Crystals showing different defects are shown in different figures shown below:
If you will see the first figure then you will observe that atoms having some impurity are present there.
Due to the presence of the impurity atom the actual atom will be missing from the lattice site. This is known as point defect. By using some technique of purifying the crystal these impurities can be separated out from the crystal.