Intrinsic Semiconductors can be an element or compound semiconductor.
During conduction in semiconductors electron move to the positive terminal while holes move to the negative terminal. Therefore, current flow in intrinsic semiconductors is due to the movement or motion of thermally generated electrons and holes that move in pairs.
The number of charge carriers per unit volume is known as intrinsic carrier concentration.
ni = N exp(-Eg / 2KbT)
In the above formula:
ni = Intrinsic Carrier Concentration
N = a constant for a given semiconductor
Eg = Energy Gap in Joules
Kb = Boltzmann Constant = 1.38E-38 J/K
T = Temperature in Kelvin
To solve the problem of having a semiconductor with a very large number of electrons moving through positively charged fixed ions or holes we make use of crystalline semiconductors because of their symmetry and order.
Therefore, the electrons move through a well defined periodically arranged distribution instead of random distribution of ions.
Recombination is a process that occurs when an electron loses energy and falls from the conduction band to a hole in the valence band.
Extrinsic Semiconductors are those semiconductors that are doped with controlled amount of impurities.
There are two types of extrinsic semiconductors:
1. n - type semiconductors
2. p - type semiconductors
N - Type Semiconductors are those semiconductors that are doped with a pentavalent (Group 5) element - that is an element that has 5 electrons in its outermost shell.
In the n - type semiconductor, the majority carriers are the electrons while the minority carriers are the holes. Also, the concentration of electrons in the n - type material is far greater than its intrinsic concentration.
Also again, the concentration of holes in an n - type semiconductor is smaller when compared to an intrinsic material because the larger number of electrons present increases the recombination rate.
When pentavalent impurities are added to a semiconductor, an allowable energy level is created just below the conduction band.
P - Type Semiconductors are those semiconductors that are doped with a trivalent (Group 3) element - that is an element that has 3 electrons in its outermost shell.
In the p - type semiconductor, the majority carriers are the holes while the minority carriers are the electrons.
When trivalent impurities are added to the intrinsic semiconductor, an allowable discrete energy level is created just above the valence band.
Under thermal equilibrium, the product of free negative and positive concentration is a constant independent of the amount of donor and acceptor impurity doping. This relationship is known as Mass Action Law.
Equation for Mass Action Law : np = ni2
