Extrinsic Semiconductors
An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors, etc.) or optoelectronic applications (light emitters and detectors).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.
P – Type Semiconductors: A P – type semiconductor is an intrinsic semiconductor (like Si) in which an impurity acting as an acceptor (like e.g. boron B in Si) has been intentionally added. These impurities are called acceptors since once they are inserted in the crystalline lattice, they lack one or several electrons to realize a full bonding with the rest of the crystal.
From the above figure, we see that a p-type semiconductor has a lower electron density n and a higher hole density p than the same intrinsic semiconductor. Electrons are said to be the minority carriers whereas holes are the majority carriers.
For extrinsic semiconductors, the dopant density is always far higher than the intrinsic carrier density: NA>>ni. In the case of a p-type material, the hole density is then close to the dopant density NA.
N – Type Semiconductors: A N – Type semiconductor is an intrinsic semiconductor (e.g. silicon Si) in which a donor impurity (e.g. arsenic As in Si, or Si in GaAs) has been intentionally introduced. The impurities are called donor impurities since they have to give an extra electron to the conduction band in order to make all the bonds with neighbouring atoms (As is pentavalent while Si is tetravalent).
From the above figures, we see that a n-type semiconductor has a higher electron density n and a lower hole density p than the same intrinsic semiconductor. Holes are said to be the minority carriers whereas electrons are the majority carriers.