EC and EV are the energies of conduction band minimum and valence band maximum. ey are determined by chemical bonds of atoms in the crystal lattice, and are measured with respect to a reference level called the vacuum level representing the energy level corresponding to a free fugitive electron that has escaped from the semiconductor. e electron anity χs of the semiconductor is the energy dierence between the conduction band edge EC and the vacuum energy level E0, or in symbols,
χs C= −E E0 (6.1)
e work function φs of the semiconductor is the dierence in energies between vacuum level and Fermi level, expressed as
φS F= −E E0 (6.2)
e Fermi level EF, a hypothetical level of potential energy for an electron inside a crystalline solid, is dened as the energy level at which the probability of being occupied by an electron is 50%. e position of the Fermi level with respect to the band edges denes the mobile charge occupancy of the bands. e nearer the Fermi level is to the conduction band, the more free electrons are present in that band. Similarly, the less the distance between the Fermi level and the valence band, the more holes are present in this band.