Band Theory of Semiconductors

The band theory of semiconductors relies on the valence band and conduction band. It’s additionally referred to as the band theory of solids or zone theory of solids. It describes  conductors, semiconductors and insulators very clearly and distinctly. 

Moreover, Plotting the available energies for electrons in the materials is a good approach to see the difference between conductors, insulators, and semiconductors. The possible energy levels form bands rather than distinct energies as in the case of unbound atoms.

Before understanding the band theory, we would have to possess information of the subsequent terms:

Valence Band- It’s created from those valence shell orbitals that have electrons in them. As an example, a metal valence band is formed from 3s¹ orbital. The electronic configuration of metal is 1s², 2s² 2p⁶, 3s¹. 

Conductivity Band- It’s created from those orbitals that are unoccupied by electrons either in the valence shell or higher unoccupied shell. Thus, the orbitals of the conductivity band are empty. Again, let’s take the example of sodium as after orbital 3s next orbital 3p is empty thus it forms a conductivity band. Whereas the next accessible band within the energy structure that is empty is called the conductivity band.  The band structure of metal may be shown as follows:

Forbidden Gap – The gap or energy distinction between the valence band and conductivity band is termed the prohibited gap.  

Conductors – Those materials which permit electricity to go through them are referred to as conductors. Examples – copper, iron, zinc etc.  

Semiconductors – Those materials that show conduction between conductors and insulators are referred to as semiconductors. Examples – Si, Ge, As etc.  

Insulators – Those materials that are not to enable electricity to go through them are referred to as insulators. Examples – wood, glass, stones etc. 

Band Theory

In chemistry, according to the band theory of solids electrons jump from valence band to conduction band even at standard temperature and if this happens then the solid conducts electricity. Conduction depends on the gap between the valence band and conduction band. If the gap or energy difference between the valence band and conduction band is over or adequate to 5eV then the fabric can behave as a dielectric. If the energy difference between the valence band and conductivity band is up to or less than 3ev then the material or solid is termed a semiconductor. If the valence band and conduction band overlap one another then the solid is called conductors. The reasoning behind it’s that electrons can jump from valence band to conduction band and therefore conduct electricity. Whereas if the gap is just too much between the valence band and conduction band (more than or adequate to 5ev) then electrons can’t jump from the valence band to the conduction band, therefore these materials or solids can’t conduct electricity. If the valence band and conduction band are neither overlapping nor at too far distance, then a couple of electrons might jump from the valence band to the conduction band and these materials are referred to as semiconductors.

SEMICONDUCTORS

Semiconductors are defined to possess conductivity in between a nonconductor and a conductor. Because of this property, semiconductors are quite common on a daily basis in electronics since they probably won’t short circuit like a conductor. They get their characteristic conductivity from their small band gap. Having a band gap prevents short circuits since the electrons are not continuously within the conduction band. A little band gap permits for the solid to possess a strong enough flow of electrons from the valence to conduction bands in order to possess some conductivity. Electrons within the conduction band become free from the nuclear charge of the atom and therefore can move freely around the band. Thus, this free-moving electron is known as a negative charge carrier since having the electron during this band causes electrical conductivity of the solid. When the electron goes away from the valence band, the state then becomes a positive charge carrier, or a hole.

Type of Semiconductors

There are two types of semiconductor

• Intrinsic Semiconductor

It is a pure semiconductor, whose characteristics are completely determined by the material itself. The number of electrons in the conduction band is equal to the number of holes in the valence band in this case. These semiconductors are also referred to as i-types.

• Extrinsic Semiconductor

It is impure semiconductors that have been “doped” to increase conductivity. Extrinsic semiconductors are classified into two types: p-type and n-type. To pull electrons from the valence band, a “dopant” atom is introduced to the lattice. This atom is known as an acceptor.

As additional acceptors are added to the lattice, the number of holes begins to outnumber the number of negative charge carriers, resulting in a p-type (positive type) semiconductor. Donors, or “dopant” atoms that donate electrons to the conduction band, are abundant in N-type semiconductors.

CONCLUSION

Electrical properties of all the solid compounds are considered by excitation energy of charge careers.  Metals and insulators are the very least examples of solid state materials. Solid band theory is applicable just for solids with covalent and metal bonds.According to this semiconductors can extremely act as insulators at absolute zero. when this temperature and however still staying below the melting of the solid, the metal would act as a semiconductor.