People have understood the basic knowledge that semiconductor materials can produce light 50 years ago, and the first commercial diode was produced in 1960.
The structure of LED is mainly composed of PN junction chip, electrode and optical system. When a forward bias is applied to the electrode, electrons and holes are injected into the P and N regions respectively. When the non-equilibrium minority carriers recombine with the majority carriers, the excess energy will be converted into light energy in the form of radiated photons.
In the 1980s, LEDs were mainly used for display devices and short-distance, low-speed optical fiber communication light sources, such as the display of characters, numbers and other symbols of various instrument indicators. Due to reasons such as brightness and color, LED can not completely replace the general lighting source, and this is a very important development direction of LED in the future. The basic structure of the LED is a piece of electroluminescent semiconductor material, placed on a leaded shelf, and then sealed with epoxy resin around it to protect the inner core wire, so the LED has good shock resistance. The schematic diagram of the LED structure is shown in Figure 1.
The area of the LED luminous body chip is 10.12mil2 (1mil=0.0254mm2). At present, large-chip LEDs have been developed internationally, with a chip area of 40mil2. The light-emitting process of LED includes three parts: carrier injection under forward bias, recombination radiation and light energy transmission. The tiny semiconductor chip is encapsulated in a clean epoxy resin. When electrons pass through the chip, the negatively charged electrons move to the positively charged hole area and recombine with it. The electrons and holes disappear while generating photons. The greater the energy (band gap) between the electron and the hole, the higher the energy of the photon produced. The energy of photons in turn corresponds to the color of light. In the spectrum of visible light, blue light and purple light carry the most energy, while orange light and red light carry the least energy. Because different materials have different band gaps, they can emit different colors of light.
Read more: Color rendering of light source
The energy states of electrons and holes in different semiconductor materials are different, and the amount of energy released when the electrons and holes are recombined is also different. The more energy released, the shorter the wavelength of the emitted light. Commonly used are LEDs that emit red, green or yellow light. Phosphorous arsenic like LEDs emit red light, phosphorized like LEDs emit green light, and silicon carbide LEDs emit yellow light.
The forward volt-ampere characteristic curve of the LED is very steep, and a current-limiting resistor must be connected in series to control the current through the LED. In a DC circuit, the current-limiting resistance R can be calculated by the following formula
R=（E－VF/lF） (1-1)
Where: E is the power supply voltage; VF is the forward voltage drop of the LED; lF is the general operating current of the LED.
In an AC circuit, the current-limiting resistance R can be calculated by the following formula
R=(Ve－VF)/2IF (1-2)
Where: Ve is the effective value of the AC power supply voltage.