Fermi level is the measure of:

The Fermi level is a concept in solid-state physics that is used to describe the probability of occupancy of electrons or holes in a material. It represents the energy level at which the probability of finding an electron is 50%.

Explanation:

In order to understand the Fermi level, we need to understand the concept of energy bands in solids. Solids are made up of atoms that are closely packed together. When these atoms come together to form a solid, their energy levels combine to form energy bands.

- Energy Bands: Energy bands are ranges of energy levels that electrons can occupy in a solid. The two most important energy bands are the valence band and the conduction band. The valence band is the band that contains the electrons that are bound to the atoms and are not free to move. The conduction band is the band that contains the electrons that are free to move and participate in electrical conduction.

- Fermi-Dirac Distribution: The distribution of electrons in these energy bands is determined by the Fermi-Dirac distribution function. This function describes the probability of occupancy of energy levels by electrons or holes at a given temperature. It is characterized by a parameter called the Fermi-Dirac distribution function, denoted by f(E), which represents the probability of finding an electron at a particular energy level E.

- Fermi Level: The Fermi level is a special energy level within the energy band structure of a solid. It represents the energy at which the probability of finding an electron is 50%. At absolute zero temperature, the Fermi level corresponds to the highest filled energy level in the valence band. As temperature increases, the Fermi level moves towards the conduction band, allowing more electrons to occupy higher energy levels.

- Probability of Occupancy: The Fermi level is directly related to the probability of occupancy of electrons or holes in a material. At the Fermi level, the probability of occupancy is highest, and it decreases as we move away from the Fermi level in either direction.

Therefore, the correct answer is option B: Probability of occupancy of electrons or holes. The Fermi level represents the energy level at which the probability of finding an electron is 50%. It is a fundamental concept in solid-state physics that helps us understand the behavior of electrons in materials and their ability to conduct electricity.