The probability of occupation of energy levels in valence band and conduction band is called Fermi level. As the temperature increases free electrons and holes gets generated. In intrinsic semiconductor, the number of holes in valence band is equal to the number of electrons in the conduction band. Hence, the probability of occupation of energy levels in conduction band and valence band are equal. Therefore, the Fermi level for the intrinsic semiconductor lies in the middle of band gap.