Absorption of energy by the semiconductor can boost electrons from the valence band to the conduction band. The amount of energy is at least as large as the energy gap (Eg). Ideally, the conduction band is completely empty at T = 0 K. As the temperature is raised, some electrons get enough energy to "pop up" into the conduction band, in accordance with statistical physics. Therefore, higher energy states can be populated be electrons due to thermal excitation