Explain the formation of energy bands in solids. With neat energy band diagrams explain extrinsic semiconductors.

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Formation of energy bands:-

In case of a single atom, there is a single energy for an electron orbit.
However, when two atoms are brought close to each other, it leads to intermixing of electrons in the valence shell.

As a result, the number of permissible energy levels are formed, which is called an energy band.

Extrinsic Semiconductors:-

a)P-type semiconductor:

P-type semiconductors are formed when a semiconductor like silicon is doped with a tri-valentimpurity.
The tri-valent impurity enters the lattice of tetra-valent semiconductor atoms.
The impurity atom forms the bond with three semiconductor atoms.
As a result, the bond with the fourth neighbour is not complete.
This bond can be filled with another electron from an other bond, and the vacancy can be transported to the other bond.
This vacancy of electrons is called a hole.
As shown in the band diagram, the impurity level is created a little above the valence band.
With a little gain in energy, a valance electron can cross over to this level and leave behind the hole which is responsible for conduction.

b)N-type semiconductor:-

N-type semiconductors are formed by doping penta-valent impurities into tetra-valent semiconductors.
The impurity atoms form bonds with four neighbouring semiconductor atoms.
The fifth electron of the impurity atom remains unbonded.
This electron jumps into the conduction band, since the impurity level is close to the conduction band as shown in the figure.
These electrons are ressponsible for conduction.

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