Like ordinary diodes, light-emitting diodes consist of a PN junction and also have unidirectional conductivity. When a forward voltage is applied to the light-emitting diode, the holes injected from the P region to the N region and the electrons injected from the N region to the P region are respectively connected with the electrons in the N region and the empty holes in the P region within a few microns near the PN junction. Hole recombination, resulting in spontaneous emission of fluorescence. The energy states of electrons and holes in different semiconductor materials are different. When electrons and holes recombine, the energy released is somewhat different, the more energy released, the shorter the wavelength of the light emitted. Commonly used are diodes that emit red, green or yellow light. The reverse breakdown voltage of the light-emitting diode is greater than 5 volts. Its forward volt-ampere characteristic curve is very steep, and a current limiting resistor must be connected in series to control the current through the diode.

The core part of the light-emitting diode is a wafer composed of P-type semiconductors and N-type semiconductors. There is a transition layer between the P-type semiconductors and the N-type semiconductors, which is called a PN junction. In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, the excess energy is released in the form of light, thereby directly converting electrical energy into light energy. When the reverse voltage is applied to the PN junction, it is difficult for minority carriers to inject, so it does not emit light. When it is in a forward working state (that is, a forward voltage is applied to both ends), when the current flows from the LED anode to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared, and the intensity of the light is related to the current.