Question

What is meant by the term forbidden zone in reference to the band theory of solids?

Answer 1

Atoms are arranged in a systematic space lattice in solids. Solids are classified into conductors, semi-conductors and insulators on the basis of the difference in the energy gap between valence band and conduction band.

Complete step by step solution:
Semiconductors have resistivity between good conductors and insulators. The discrete energy levels in an isolated atom are called energy bands.
In solids, there is a valence band and conduction band. The conduction band has higher energy. These bands are arranged in the increasing order of energy which is given below:
Valence bandThe energy gap between the valence and conduction band is called the forbidden zone. Forbidden zone is very large in insulators. Its value is more than ${\text{3eV}}$. There is no electron available for conduction, i.e. it has no electron to jump from valence band to conduction band. The large gap denotes that it requires a large amount of energy to jump.
Now we can take the case of conductors. There is no forbidden zone, i.e. both of the bands overlap. From the valence band, the electrons can move freely to the conduction band.
The forbidden gap is small in the case of semiconductors. So only a small energy can be applied for the electrons to jump from valence band to conduction band.

Note: Forbidden zone is around $0.7{\text{eV}}$ for germanium, but the forbidden zone for silicon is around ${\text{1}}{\text{.1eV}}$. In a pure semiconductor, at absolute zero temperature, the valence band is completely filled and the conduction band is vacant. Some of the electrons get energy to break the covalent bond and move into the conduction band in room temperature.