In a semiconductor, the valence band is filled with electrons, while the conduction band is typically empty. When the energy bands are close enough, an electron can "tunnel" through the forbidden energy gap, moving directly from the valence band to the conduction band. This process is governed by Quantum Mechanics and is described by the Schrödinger equation. The probability of tunneling depends on factors like the width and height of the energy barrier and the effective mass of the electron.
