Semiconductors are amorphous or crystalline solids that have a conductivity between a conductor and an insulator either due to an impurity (extrinsic semiconductors) or because of temperature change. They are mainly classified into two types:
- Intrinsic semiconductors
- Extrinsic semiconductors
We have read about the intrinsic semiconductor before and we know that the conductivity of an intrinsic semiconductor depends upon the surrounding temperature and also that at room temperature, its conductivity is very low. With such low conductivity, it is considered unsuitable for use in the electronic devices. In order to deal with this problem, we introduce the concept of doping and extrinsic semiconductors. In this section, we will learn about extrinsic semiconductors.
Extrinsic semiconductors are semiconductors that are doped with a specific impurity. The impurity, in turn, modifies the electrical properties of the semiconductor and makes it more suitable for use in electronic devices such as diodes and transistors. In this process, a small amount of any suitable impurity is added to a pure material, increasing its conductivity by many times. Extrinsic semiconductors are also called impurity semiconductors or doped semiconductors; the process of adding impurities is deliberately termed as doping and the atoms that are used as an impurity are termed as dopants.
The dopant added to the material is chosen such that the original lattice of the pure semiconductor is not distorted. Also, the dopants occupy only a few of the sites in the crystal of the original semiconductor and it is necessary that the size of the dopant is nearly equal to the size of the semiconductor atoms.
In the process of doping on a material such as a tetravalent Si or Ge, two types of dopants are used:
- Pentavalent atoms, that is, atoms with valency 5; such as Arsenic (As), Phosphorous (Pi), Antimony (Sb), etc.
- Trivalent atoms, that is, atoms with valency 3; such as Indium (In), Aluminium (Al), Boron (B), etc.
When a tetravalent atom such as Si or Ge is doped with a pentavalent atom, it occupies the position of an atom in the crystal lattice of the Si atom. The four of the electrons of the pentavalent atom bonds with the four neighbouring silicon atoms and the fifth one remains weakly bound to the parent atom. As a result of this, the ionization energy required to set the fifth electron free is very less and the electrons become free to move in the lattice of the semiconductor. Such semiconductors are termed as n-type semiconductors.
When a tetravalent atom such as Si or Ge is doped with a trivalent impurity such as Al, B, In, etc., the dopant atom has one less electron than the surrounding atoms of Si or Ge. Thus, the fourth atom of the tetravalent atom is free and a hole or a vacancy is generated in the trivalent atom. In such materials, the holes are the charge carriers and such semiconductors are termed as p-type semiconductors.
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