CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
TYPES OF SEMICONDUCTORS
Elemental semiconductors: Si and Ge
- Inorganic: CdS, GaAs, CdSe, InP, etc.
- Organic: anthracene, doped pthalocyanines, etc.
- Organic polymers: polypyrrole, polyaniline, polythiophene, etc.
TYPES OF SEMICONDUCTORS BASED ON PURITY
A semiconductor in its pure state is called intrinsic semiconductor.
In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take share of one electron from each such neighbour.
Effect of temperature on conductivity of semiconductors
As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break away (becoming free electrons contributing to conduction).
The thermal energy ionises only a few atoms in the crystalline lattice and creates a vacancy in the bond. The neighbourhood, from which the free electron (with charge –q) has come out leaves a vacancy with an effective charge (+q ). This vacancy with the effective positive electronic charge is called a hole.
An intrinsic semiconductor will behave like an insulator at T = 0 K.
At higher temperatures (T > 0K), some electrons are excited and move from the valence band to the conduction band.
In intrinsic semiconductors, the number of free electrons, ne is equal to the number of holes, nh. That is,
Where, ni is called intrinsic carrier concentration.
Both electrons and holes move to conduct.
Also, I = Ie + Ih, where Ie is the current due to movement of free electrons and Ih is the current due to movement of holes.
A semiconductor doped with suitable impurity, is called extrinsic semiconductor. The deliberate addition of a desirable impurity is called doping and the impurity atoms are called dopants.
On the basis of doped impurity extrinsic semiconductors are of two types, i.e., m-type and p-type.