NEET Chemistry Notes Solid State – Defects or Imperfections in Solids
Defects or Imperfections in Solids
Defects or Imperfections in Solids
Any departure from perfectly ordered arrangement of atoms in a crystal is called imperfection or defect.
If imperfection in crystals are such that the ratio between the cations and the anions remain the same as described in its molecular formula, the defect will be called stoichiometric defects.
When the ratio of cations and anions, due to imperfection, differ from that indicated by their molecular formula, the defects are called non-stoichiometric defects. These defects result in either excess of metal atom or excess of non-metal atom.
F-centre A negative ion may be missing from its lattice site, leaving a hole which is occupied by an electron, thereby maintaining the electrical balance. The electrons, thus trapped in the anion vacancies are called F-centres because they are responsible for imparting colour to the crystals.
Addition of small amount of foreign impurity in the host crystal is known as doping. It increases the electrical conductivity.
Another common method of introducing defects in ionic solids is by adding impurity ions having different charge than host ion. These foreign atoms are present at lattice site in substitutional solids and at vacant interstitial sites in interstitial solids.
These defects are important in n-type and p-type semiconductors.
n-type semiconductors Group 14 elements when doped with group 15 elements they are called n-type semiconductors.
p-type semiconductors Group 14 elements when doped with group 13 elements are called p-type semiconductors.
Properties of Solids
Electrical Properties of Solids
- Solids are classified into three groups on the basis of conduction power, namely
- Conductors with conductivity range of the order of 107 e.g. metals.
- Semiconductors with conductivity range 106 -10-4 e.g. semi-metals.
- Insulators with conductivity range 10-10 -10-20 e.g. non-metals.
In conductor thjere is no energy gap between VB and CB. So electron transfer can take place easily and it affects electrical conductivity
In semiconductor energy gap between VB and CB is small, so some electron may jump from VB to CS due to its conductivity observed. Conductivity of semiconductor is proportional to temperature.
Semiconductors one classified into following types:
- Intrinsic Semiconductors
Here, heat is responsible for rejection of electrons thus, leaving a (+)ve charge (positive hole). The crystal can now conduct electricity because on applying the electric field the electrons and holes migrate in opposite directions. This type of conductance is called intrinsic conductance and the semiconductors showing this
type of conductivity are called undoped or intrinsic semiconductors, e.g. Si, Ge, etc.
- Extrinsic Semiconductors
For increasing conductivity of pure Si and Ge, it is doped with element 13 and 15 group (periodic table). Such type of conductance is called extrinsic conductance. The extrinsic semiconductor can be of two types, viz, n-type semiconductors and p-type semiconductors.
- n-Type Semiconductor
The group 15 elements have 1 electron excess to Si or Ge, after forming 4 covalent bonds with group 14 member (Si or Ge). This excess free electron is responsible for electrical conductivity in them. Group 14 elements when doped with group 15 elements they are called n-type semiconductors. Here, n specifies that negative charge flows in them.
- p-Type Semiconductor
The group 13 elements have 1 electron short to group 14 elements, thus giving rise to electron deficient band or a hole. Here, such holes are responsible for electrical conductivity. Thus, group 14 elements when doped with elements of group 13 are called p-type semiconductors.
Here, p specifies that conduction is passed through positive holes in them.
In case of insulators, the energy gap between valence band and conduction band is so large that it cannot even covered up by supplying energy in the form of heat.