|A charge coupled device converts light (photonic energy) into electric energy. What is the underlying mechanism that makes this happen? — PM, Belfast, Ireland
As in any photoelectric cell, the energy from a single particle of light—a photon—is used to raise the energy of an electron in a diode and to propel that electron from one side of the diode to the other. In this process, the light energy is partly converted to electrostatic potential energy and partly to thermal energy. Since a diode only carries current in one direction, the electron is unable to return to its original side. In a photoelectric cell, the electron flows through a circuit to return to the other side of the diode and provides energy to that circuit. In a charge coupled device, a complicated charge shifting system transfers the electrons to a detector that registers how much light was absorbed.