When electrons and holes recombine, it can radiate visible light, so it can be used to make light-emitting diodes. Used as indicator lights in circuits and instruments, or composed of text or digital displays. Gallium arsenide diodes emit red light, gallium phosphide diodes emit green light, silicon carbide diodes emit yellow light, and gallium nitride diodes emit blue light. Due to chemical properties, it is divided into organic light-emitting diode OLED and inorganic light-emitting diode LED.
Light-emitting diodes are commonly used light-emitting devices that emit energy through the recombination of electrons and holes to emit light. They are widely used in the field of lighting.  Light-emitting diodes can efficiently convert electrical energy into light energy and have a wide range of uses in modern society, such as lighting, flat panel displays, and medical devices. 
This kind of electronic components appeared as early as 1962. In the early days, they could only emit low-luminance red light. Later, other monochromatic versions were developed. The light that can be emitted today has spread to visible light, infrared and ultraviolet light, and the luminosity has also increased to a considerable extent. The luminosity. The use has also been used as indicator lights, display panels, etc.; with the continuous advancement of technology, light-emitting diodes have been widely used in displays and lighting.
Like ordinary diodes, light-emitting diodes are composed of a PN junction, and they also have unidirectional conductivity. When the forward voltage is applied to the light-emitting diode, the holes injected from the P area to the N area and the electrons injected from the N area to the P area are respectively in contact with the electrons in the N area and the voids in the P area within a few microns of the PN junction. The holes recombine and produce spontaneous emission 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 emitted light. 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 it must be used in series with a current-limiting resistor to control the current through the diode.
The core part of the light-emitting diode is a wafer composed of a P-type semiconductor and an N-type semiconductor. There is a transition layer between the P-type semiconductor and the N-type semiconductor, which is called a PN junction. In the PN junction of certain semiconductor materials, when the injected minority carriers and the majority carriers recombine, the excess energy is released in the form of light, thereby directly converting electrical energy into light energy. With reverse voltage applied to the PN junction, it is difficult to inject minority carriers, so it does not emit light. When it is in a positive working state (that is, a positive 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. The intensity of the light is related to the current.