When an electron makes a transition from a higher
energy state to a lower energy state, then all or part of the energy
difference is emitted as electromagnetic radiation. If
the excitation is due to electrical bias, the radiative transition
process is called electroluminescence.
Electroluminescence (EL) technique is widely used for examining optical properties of electronic structures. In this case, the analyzed structure is biased by applying a voltage to its electric terminals, and then the generated electron-hole pairs are separated under the applied electric field.