What is fermi level ?

Fermi level is defined as the energy state or level which has a 50% probabilis of being filled by an electron.
For pure semiconductor, the Fermi Level lies midway between conduction band and valence band asshown in fig. At an absolute temperatureT=0 K, all energy states above the fermi level are empty and all energy states below fermi level are occupied. As the temperature increases some covalent band breaks up and such electrons go to the conduction band. Due to this, the fermi level shifts upward as the temperature increases.