There are two types of Semiconductors. They are Intrinsic and Extrinsic semiconductors.
Types of Semiconductors
- Intrinsic Semiconductor
- Extrinsic Semiconductor
A semiconductor in its pure form without impurity is called an intrinsic semiconductor.
Here impurity means any other atom in the crystal lattice. Each Silicon atom has four electrons in the outermost orbit and is covalently bonded with the neighboring atoms to form the lattice.
A small increase in temperature is sufficient enough to break some of the covalent bonds and release the electrons free from the lattice.
As a result, some states in the valence band become empty and the same number of states in the conduction band will be occupied. The vacancies produced in the valence band are called holes.
As the holes are deficient in electrons, they are treated to possess positive charges. Hence, electrons and holes are the two charge carriers in semiconductors. In intrinsic semiconductors, the number of electrons in the conduction band is equal to the number of holes in the valence band.
The conduction is due to the electrons in the conduction band and holes in the valence band.
These currents are represented as Ie and Ih respectively.
The total current (I) is always the sum of the electron current (Ie) and the hole current (Ih). I = Ie + Ih .
An intrinsic semiconductor behaves like an insulator at 0 K. The increase in temperature increases the number of charge carriers (electrons and holes).
The intrinsic carrier concentration is the number of electrons in the conduction band or the number of holes in the valence band in an intrinsic semiconductor.
When a carrier concentration is not enough in an intrinsic semiconductor, some impurities are added to increase the carrier concentration. The process of adding impurities to the intrinsic semiconductor to increase the carrier concentration is called Doping.
The impurities atoms are called dopants and are of order 100 ppm. There are two types of extrinsic semiconductors one is an n-type semiconductor and another one is a p-type semiconductor.
Both n-type and p-type semiconductors are neutral as topping neutral atoms to the intrinsic semiconductor.
An n-type semiconductor is made of doping pure germanium or silicon with a dopant from group V pentavalent impurities (element, compound, or mixture) such as Phosphorus, Arsenic, and Antimony.
The majority carriers of current in an n-type semiconductor are electrons and the minority carriers are holes.
A semiconductor doped with a pentavalent impurity is called an n-type semiconductor.
Germanium or Silicon substrate are doped with trivalent atoms from group-iii elements such as Boron, Aluminium, Gallium and indium.
An extrinsic semiconductor where the holes are the majority carriers and thermally generated electrons are minority carriers is called a P-type semiconductor.