This electronic component appeared as early as 1962. In the early stage, it could only emit red light with low luminosity. Later, other monochromatic versions were developed. The light that can be emitted has spread all over visible light, infrared light and ultraviolet light, and the luminosity has also been improved to a relatively high luminosity. It is also used as indicator light and display board at the beginning;

With the continuous progress of technology, LED has been widely used in displays and lighting.

Like ordinary diodes, light emitting diodes are composed of a PN junction and also have single 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 recombine with the electrons in the N region and the holes in the P region within a few microns near the PN junction, respectively, to generate spontaneous emission fluorescence.

The energy states of electrons and holes in different semiconductor materials are different. When electrons and holes recombine, the amount of energy released is different. The more energy released, the shorter the wavelength of light emitted. Diodes that emit red, green or yellow light are commonly used. The reverse breakdown voltage of the LED is greater than 5 V. Its forward volt ampere characteristic curve is very steep, so the current limiting resistor must be connected in series to control the current passing through the diode.

The core of LED is a chip composed of P-type semiconductor and N-type semiconductor. There is a transition layer between P-type semiconductor and N-type semiconductor, called PN junction. In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, the excess energy will be released in the form of light, thus directly converting electrical energy into optical energy.

With reverse voltage applied to the PN junction, it is difficult to inject a few carriers, so it does not emit light. When it is in a positive working state (that is, a positive voltage is applied at both ends), when the current flows from the LED anode to the cathode, the semiconductor crystal will emit light of different colors from ultraviolet to infrared. The intensity of light is related to the current.