Question

What is a Fermi energy level? What is its position in case of an intrinsic semiconductor?

Answer 1

Hint : Fermi energy level is related to the maximum energy level that can be occupied by an electron at absolute zero temperature. For an intrinsic semiconductor, the number of holes in the valence band is equal to the number of electrons in the conduction band.

Complete step by step answer
In a material at absolute zero, all the electrons will have the lowest energy possible and they will occupy the lowest possible energy state available to them. The Fermi level in this case is associated with the highest occupied energy level beyond which no electron can exist. Fermi energy level is very important in dealing with the electrical and thermal properties of solids.
At non-zero temperatures, the electrons that have energy above the Fermi energy of the solid will then contribute to the electrical conductivity of the material. For higher temperatures, more electrons will have energies above the Fermi level.
In an intrinsic or pure semiconductor, the number of holes in the valence band is equal to the number of electrons in the conduction band. Hence, the probability of occupation of energy levels by electrons in the conduction band and holes in the valence band is equal. Therefore, the Fermi level for the intrinsic semiconductor lies in the middle of the forbidden band of the semiconductor that is between the conduction band and the valence band. In this region of energy, no electrons or holes can exist.

Note
Fermi level and Fermi energy are usually used interchangeably. Although, both the terms are equal at absolute zero temperature, they are distinct at other temperatures. The Fermi energy is only defined at absolute zero, while the Fermi level is defined for any arbitrary temperature.