Semiconductor-Part-ii : Semiconductor-Part-ii Intrinsic Semiconductors
Electric current in intrinsic semiconductor.
Electrical conductivity of semiconductor.
Extrinsic semiconductors.
Types of Extrinsic Semiconductors.
Difference between Intrinsic and Extrinsic Semiconductors.
Intrinsic Semiconductors : Intrinsic Semiconductors A pure semiconductor is called intrinsic semiconductor. It has thermally generated current carriers. Examples of pure or intrinsic semiconductor used frequently are germanium and silicon.
Intrinsic semiconductor on the basis of Valence Bond theory : Intrinsic semiconductor on the basis of Valence Bond theory At o K all the covalent bonds are complete. Therefore, no free electron is available in the crystal for the conduction of current. Hence silicon crystal behaves as an insulator at o K.
At room temperature, a covalent bond breaks, an electron becomes free. The electron which leaves the bond is called free electron and the vacancy created in the covalent bond due to the release of the electron is called a hole.
Intrinsic Concentration : Intrinsic Concentration ne=nh=ni
Ne –Number of free electrons per unit volume.
Nh-Number of holes per unit volume.
Ni-number density of intrinsic carries
The semiconductor in which the current carries holes and electrons are created due to only thermal excitation across the forbidden energy gap is called an intrinsic semiconductor(On the basis of energy band theory).
Extrinsic Semiconductors : Extrinsic Semiconductors The process of adding impurities in the intrinsic semiconductor is called doping. The impurities added in the intrinsic semiconductor to increase its conductivity are known as dopants.
A semiconductor obtained after adding impurity atoms in the intrinsic semiconductor is called extrinsic or doped semiconductor.
Types of impurities : Types of impurities Pentavalent impurity-The elements whose each atom has five valence electrons are called pentavalent impurities. For example Arsenic,Antimony,Phosphorous etc.
Trivalent impurity-The elements whose each atom has three valence electrons are called trivalent impurities. For example, Indium, Gallium, Aluminium etc.
P-type semiconductors : P-type semiconductors When trivalent impurity is added to pure germanium or silicon crystal, we get extrinsic semiconductor known as p-type semiconductor. Majority charge carriers in p-type semiconductor are holes and minority carriers are electrons which are thermally generated. Since each trivalent impurity atom accepts one electron from the neighboring silicon atom, so it is known as acceptor impurity.
N-type semiconductor : N-type semiconductor When pentavalent impurity atoms are added to pure germanium or silicon crystal, we get an extrinsic semiconductor known as n-type semiconductor.
Majority charge carriers in n-type semiconductor are electrons and the minority charge carriers are holes which are thermally generated. Since each pentavalent impurity atom donates one electron to the crystal ,so it is known as donor impurity.
Number densities of majority and minority carriers in extrinsic semiconductor : Number densities of majority and minority carriers in extrinsic semiconductor ne=nh=ni²
nh>ne. In p-type semiconductor.
ne>nh. In n-type semiconductor.
Electrical conductivity of semiconductor : Electrical conductivity of semiconductor I=A e(nhvh+neve)
I/A=e(nhvh+neve)
J=e(nhvh+neve)
σE=e(nhvh+neve)
σ=e(nhµh+neµe)
1/σ=ρ=1/e(neµe+nhµh)
Where µh-mobility of holes and µe-mobility of eletrons.µh=vh/E and µe=ve/E
Difference between intrinsic and extrinsic semiconductors : Difference between intrinsic and extrinsic semiconductors Intrinsic semiconductors Extrinsic semiconductors 1.It is the crystals of pure elements like Si and Ge.
ne=nh
Conductivity is low.
Conductivity of intrinsic semiconductors mainly depends on their temperature. It is impure elements.
ne≠nh
Conductivity is high.
Conductivity depends on the temperature as well as the amount of impurity added in them.