Like semiconductor lasers, semiconductor light emitting diodes are also PN junctions, which emit light by injecting electrons into the PN junction from an external power source. The semiconductor light emitting diode, as LED, is composed of P layer formed by P-type semiconductor, N layer formed by N-type semiconductor, and active layer formed by double heterostructure in the middle. The active layer is a luminous area with a thickness of 0.1 ～ 0.2 μ M or so. What is the light-emitting mechanism of semiconductor light-emitting diodes?

Atoms, molecules and some semiconductor materials can respectively absorb and emit light or electromagnetic waves of a certain wavelength. According to the solid energy band theory, the energy states of electrons in semiconductors are divided into valence band and conduction band. When electrons transition (transfer) from energy state E1 in one band to energy state E2 in another band, they will emit or absorb a certain frequency（ υ) Light. υ Difference with energy（ Δ E=E2-E1) is proportional, i.e

υ=Δ E/h (Hz)

This equation is called Bohr condition. Where h=6.626 × 10-34J·s。 When the light-emitting diode works, under the positive bias, the empty conduction band of the semiconductor is usually occupied by the electrons injected into it through the junction. These electrons recombine with the holes on the valence band and emit photons, which generates light.

The photon energy emitted is approximately the band gap energy between the conduction band and the valence band of a specific semiconductor. This natural emission process is called spontaneous emission recombination. Obviously, radiative transition is the basis of composite luminescence. The recombination of injected electrons may also be non luminous, that is, non radiative recombination.

In the case of non radiative recombination, the energy lost by the conduction band electrons can be changed into multiple phonons, which will heat the crystal. This process is called multiphonon transition; It can also be combined with the valence band hole to transfer the energy to another electron in the conduction band to keep it in the high energy state, and then transfer the excess energy to the lattice through the thermal balance process, which is called Auger recombination.