In our article on semiconductors, we discussed that semiconductors are amorphous or crystalline solids that have a conductivity between that of a conductor and an insulator, either due to the presence of an impurity (extrinsic semiconductors) or because of temperature change. They are mainly classified into two types as follows:
- Intrinsic semiconductors
- Extrinsic semiconductors
The conductivity of an intrinsic semiconductor depends on the surrounding temperature. At room temperature, it exhibits a low conductivity. Due to its low conductivity, it is deemed unsuitable for the use in electronic devices. In order to deal with this problem, the concept of doping arose, and as a result of which extrinsic semiconductors were manufactured. In this section, we will discuss what extrinsic semiconductors.
Why are Semiconductors Doped?
Extrinsic semiconductors are semiconductors that are doped with specific impurities. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors.
While adding impurities, a small amount of the suitable impurity is added to pure material, increasing its conductivity by many times. Extrinsic semiconductors are also called impurity semiconductors or doped semiconductors. The process of adding impurities deliberately is termed as doping and the atoms that are used as an impurity are termed as dopants.
The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors.
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.
Some Commonly Used Dopants
While doping tetravalent atoms such as Si or Ge, two types of dopants are used, and they are:
- Pentavalent atoms: Atoms with valency 5; such as Arsenic (As), Phosphorous (Pi), Antimony (Sb), etc.
- Trivalent atoms: Atoms with valency 3; such as Indium (In), Aluminium (Al), Boron (B), etc.
The reason behind using these dopants is to have similar-sized atoms as the pure semiconductor. Both Silicon and Germanium atoms belong to the fourth group in the periodic table. Hence, the choice of dopants from the third and fifth group is more viable. This ensures that the size of the atoms is not very different from the fourth group. Therefore, the trivalent and pentavalent choices. These dopants give rise to two types of semiconductors as follows:
- In-type semiconductors
- p-type semiconductors
In the next section, let us discuss each of their characteristics.
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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