An ideal crystal is one which has the same unit cell containing the same lattice points throughout the whole of the crystal. However, a number of crystals reflect the absence of perfectness in the arrangement of the lattice points. These defects were studied by Frenkel (1930).
These are of the following types:
1. Point Defects: These defects arise due to a few lattice points.
They are of two types:
(a) Valency defects: If an atom or ion is missing from the lattice site, the defect in the -crystal is called valency defect.
(b) Impurity defects: This defect arises by the entry of a foreign atom or ion as impurity in lattice site. It occupies a void in the structure; the defect is called interstitial impurity.
Impurity defects lead to the semi-conductor properties of the crystals. One important application of these defects lies in the use of germanium and silicon crystals as semi-conductor in transistors. Germanium and silicon belong to the Group IVA (or 14) of the periodic table. These elements in pure state have very low electrical conductivity. However, oh adding even traces of an element belonging to Group IIIA (or 13) or VA (or 15), the electrical conductivity is greatly enhanced. This is because of free electrons available.
When small amount; of arsenic with five valency electrons is added to silicon (4 valency electrons), the fifth valency electron of arsenic remains unshared which moves through the crystal under the influence of an electric field. This gives n-type semiconductor where current is carried by the flow of negative charge.
When a small amount of boron with three valency electrons is added to silicon (4 valency electrons), it is surrounded by 7 valency electrons instead of 8. Consequently, there is a positive hole in the lattice. In order to fill this hole, an electron moves from the neighboring atom in an electric field. As such an electron deficiency is created around the atom which it leaves. This gives rise to p-type semiconductor whose positive holes move through the lattice.
2. Line defects: These defects arise due to displacement of row of lattice site-dislocations.
3. Plane Defects: These defects arise by irregularities in a plane.
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