Explain the formation of two types of extrinsic semiconductors. Also, show their energy level diagrams.
Answers
Explanation:
After some experiments, it was observed that there was an increase in the conductivity of a semiconductor when a small amount of impurity was added to it.
These materials are Extrinsic Semiconductors or impurity Semiconductors. Another term for these materials is ‘Doped Semiconductor’. The impurities are dopants and the process – Doping.
An important condition to doping is that the amount of impurity added should not change the lattice structure of the Semiconductor. To achieve this the size of the dopant and Semiconductor atoms should be the same.
Types of Dopants in Extrinsic Semiconductors
Crystals of Silicon and Germanium are doped using two types of dopants:
Pentavalent (valency 5);
like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.
Trivalent (valency 3); like Indium (In), Boron (B), Aluminium (Al), etc.
The reason behind using these dopants is to have similarly sized atoms as the pure semiconductor. Both Si and Ge belong to the fourth group in the periodic table. Hence, the choice of dopants is from the third and fifth group. This ensures that size of the atoms is not much different from the fourth group. Hence, the trivalent and pentavalent choices.
These dopants give rise to two types of semiconductors:
n-type
p-type
n-type semiconductor
An n-type semiconductor is created when pure semiconductors, like Si and Ge, are doped with pentavalent elements.
extrinsic semiconductors
when a pentavalent atom takes the place of a Si atom, four of its electrons bond with four neighbouring Si atoms. However, the fifth electron remains loosely bound to the parent atom. Hence, the ionization energy required to set this electron free is very small. Thereby, this electron can move in the lattice even at room temperature.
the ionization energy required for silicon at room temperature is around 1.1 eV. On the other hand, by adding a pentavalent impurity, this energy drops to around 0.05 eV.
It is important to remember that the number of electrons made available by the dopant atoms is independent of the ambient temperature and primarily depends on the doping level. Also, as the temperature rises, the Si atoms free some electrons and generate some holes. But, the number of these holes is very small. Hence, at any given point in time, the number of free electrons is much higher than the number of holes. Also, due to recombination, the number of holes reduce further.
In a nutshell, when a semiconductor is doped with a pentavalent atom, electrons are the majority charge carriers. On the other hand, the holes are the minority charge carriers. Therefore, such extrinsic semiconductors are called n-type semiconductors. In an n-type semiconductor,
Number of free electrons (ne) >> Number of holes (nh)
p-type semiconductor
A p-type semiconductor is created when trivalent elements are used to dope pure semiconductors, like Si and Ge. when a trivalent atom takes the place of a Si atom, three of its electrons bond with three neighbouring Si atoms. However, there is no electron to bond with the fourth Si atom.
This leads to a hole or a vacancy between the trivalent and the fourth silicon atom. This hole initiates a jump of an electron from the outer orbit of the atom in the neighbourhood to fill the vacancy. This creates a hole at the site from where the electron jumps. In simple words, a hole is now available for conduction.
It is important to remember that the number of holes made available by the dopant atoms is independent of the ambient temperature and primarily depends on the doping level. Also, as the temperature rises, the Si atoms free some electrons and generate some holes. But, the number of these electrons is very small. Hence, at any given point in time, the number of holes is much higher than the number of free electrons. Also, due to recombination, the number of free electrons reduce further.
In a nutshell, when a semiconductor is doped with a trivalent atom, holes are the majority charge carriers. On the other hand, the free electrons are the minority charge carriers. Therefore, such extrinsic semiconductors are called p-type semiconductors. In a p-type semiconductor,
Number of holes (nh) >> Number of free electrons (ne)