Explain the formation of energy bands in solids. On the basis of energy bands distinguish between a metal, a semiconductor and an insulator.
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Solution
Energy bands in solids
Band theory describes the arrangement of electrons in the solid.
The electrons have energy which is specified in a certain range.
The range of allowed energy of electrons in a solid is called energy band energy.
The band theory states about the thermodynamic and electrical properties of solid.
There are three energy bands in solids-
Valence band: It is the highest energy level associated with the electron.
Conduction band: Empty space in an atom broadens to form a band called conduction band, they are generally empty.
Forbidden band: The gap between valence band and conduction band is called Forbidden band.
When atoms are brought closer together to form solid, they combine to form molecular orbital with discrete energy level, filling of electrons in the energy band follows Pauli’s exclusion principle. These energy band are used to distinguish between a metal, a semiconductor and an insulator.
Difference between a metal, a semiconductor and an insulator.
Metals
Semiconductors
Insulators
1. Metals are good conductors of heat and electricity.
1. They are crystalline solid with conductivity lying in between metals and insulators.
1. They are poor conductors of heat and electricity.
2. In metals, conduction band overlaps the valence band.
2. In semiconductors, valence band and conduction bands are filled, and there is less energy gap between valence band and conduction bands.
2. In Insulators, the energy gap between valence band and conduction bands are high.
3. Due to overlap between the bands the movement of electrons is high therefore, it is a good conductor of electricity.
3. Due to small gap between the bands partial moment of electrons are possible. Hence, they conduct electricity.
3. Due to high gap between the bands no moment of electrons are possible. Hence, they are poor conductors.
4. Example: Copper
4. Example: Silicon
4. Example: Wood
Figure : Diagram representing energy band gaps in materials.
Hence, energy band gaps are used to distinguish between a metal, a semiconductor and an insulator.
