According to the energy band theory given by Bohr, there is always a discrete amount of energy present at different levels on the shell of an atom. It also attempts at explaining the relevance of energy band theory, which classifies different categories of energy bands into three main classes. These include – the valence band, conduction band, and forbidden band.

In this discussion, we will deal with the topic of the forbidden band and its significance depending on the role it performs in conducting electricity.

Forbidden gap

Forbidden energy gap or Forbidden gap is a gap between the ‘conduction band’ and ‘valence band’ in a given material. It is clear from the name itself that it is a forbidden band that exists with no energy. 

This is the reason why there is no electron in this type of band. However, it is important to note that valence electrons usually pass through this path while approaching the conduction band.

If in a given case, the forbidden gap is greater, then it simply implies that the valence band electrons are close to the nucleus. Weather extra external energy is still required to keep and push the electrons out of the given valence band. It must be equal to the measurements of the forbidden energy gap. 

Materials such as conductors, semiconductors, and insulators are formed depending on the size of the forbidden gap. This type of energy gap is usually huge, and valence band electrons are usually kept tightly to the atoms. However, it is likely to show some level of conduction if the temperature keeps on increasing in the case of a few insulators.

Intrinsic Semiconductors Band Gap

Now that we know what a forbidden band is, one must note that the conductivity levels given by intrinsic semiconductors are immensely dependent on the given bandgap. In such a situation, electrons with sufficient thermal energy are only available to charge the careers required for conduction to be passed across the bandgap.

There is a unique characteristic in each ‘solid material’ for an energy band structure for different energy levels. This difference in the band’s structure results in a wide-ranging electrical set of traits observed in various materials. However, in the case of insulators and semiconductors, electrons are limited to a certain number of energy bands belonging to other regions that are termed as a forbidden band. In such cases, as we have already discussed above, electrons can quickly jump from one band to another. 

In any material that has characteristics of a semiconductor, there is an intermediate size band that modifies the behaviour according to the traits of an insulator. However, there is no zero content in allowing electrons to excite into their conduction band at a given temperature that is usually set below the maximum melting point.

Types of solids in terms of the forbidden energy gap

Depending on the material, solids candy classified into three different categories –

• Semiconductors

Semiconductors are materials with no zero bandgaps and an intermediate size. These materials usually behave as insulators on the very basic level allowing electrons to generate thermal excitement into their conduction band at a given temperature.

• Conductors

These materials have great potential to transfer electric currents from one to another in a given material. There is no forbidden band between the conduction band and valence band, which gives rise to a situation where the two bands overlap. However, it is important to know that the number of free electrons found at a given room temperature is usually large in these cases. Some of the most common materials of this category include gold, aluminium, silver, and copper, as they allow an electric current to pass through them.

• Insulators

Insulators are materials with large energy gas, resulting in no conduction due to no movement of electrons from the valence band to the conduction band. Now, since there is no movement of electrons from one end to the other, electricity cannot pass through these materials as they have high resistivity with considerably low conductivity. Some of the most prominent examples of insulators include glass and wood.

Band theory of solids

The band theory of solids explains the quantum state in which electrons take place inside a metal solid. In such a situation, every molecule is composed of different discrete energy levels, making electrons move inside a molecule.

• Following Pauli’s exclusion principle, electrons are failed with specific energy orbits in atoms.

• In the case of molecules, two atomic orbitals come together to form a molecular orbital that has two different energy levels.

Conclusion

In a nutshell, one can conclude that the forbidden band is one of the three categories of gaps in a given material. This particular type of gap exists between the conduction band and valence band but has no amount of energy in it. In addition to this, One must note that no electron ever stays in this band; instead, they pass through this passage while approaching the conduction band. 

Moreover, valence band electrons are usually kept tight towards the nucleus in case the forbidden energy gap is greater. Thus, electrons always need some extra amount of external energy to put themselves out of the valence band and stay equal to the forbidden energy gas.