In semiconductors and insulators, quantum mechanics show that electrons are only found in a number of bands of energy. Electrons are forbidden from other energy levels. The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another. However, an electron needs a certain amount of energy to jump from a valence band to a conduction band. The amount of energy needed differs with different materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (heat) or a photon (light).