LED production materials and principles
Semiconductor materials were discovered 50 years ago to produce light, and the first commercial diode was produced in 1960. Known as the LED (light emitting diode), its basic structure is a piece of electroluminescent semiconductor material, placed on a leaded shelf, and then sealed with epoxy resin around it to protect the internal core wire, providing good shock resistance.
The core part of the light-emitting diode is a chip composed of p-type and n-type semiconductors. There is a transition layer between the p-type semiconductor and the n-type semiconductor, which is called a p-n 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. This kind of diode made by injection electroluminescence principle is called light-emitting diode, commonly known as LED.
When it is in a positive working state (that is, a positive voltage is applied to both ends), 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.
Types of monochromatic LEDsThe first LED light source made using the principle of semiconductor P-N junction light emission came out in the early 1960s. The material used at that time was GaAsP, which emits red light (λ p =650nm). When the driving current is 20 mA, the luminous efficiency is about 0.1 lumens/watt. In the mid 1970s, LED's were produced for green light (λ p =555nm), yellow light (introduced element of λ p =590nm) and orange light (λ p =610nm), and the luminous efficiency was also increased to 1 lumen/watt. . In the early 1980s, GaAlAs LED light sources appeared, making the light efficiency of red LEDs reach 10 lumens/watt. In the early 1990s, the successful development of two new materials, GaAlInP, which emits red and yellow light, and GaInN, which emits green and blue light, greatly improved the light efficiency of LEDs, then in 2000, the LED made by the former had a luminous efficiency of 100 lumens per watt in the red and orange zone (λ p = 615nm), while the LED made by the latter had a luminous efficiency of 50 lumens in the green zone (λ p = 530nm). Lumens/Watt.
For general lighting, white light sources are in more demand. In 1998, the white LED was successfully developed. This LED is made of a GaN chip and a yttrium aluminum garnet (YAG) package. The GaN chip emits blue light (λ p =465nm, Wd=30nm), and the Ce3+-containing YAG phosphor made by high-temperature sintering is excited by this blue light and emits yellow light with a peak value of 550nm. The blue LED substrate is installed in a bowl-shaped reflective cavity, covered with a thin layer of resin mixed with YAG, about 200-500nm. Part of the blue light emitted by the LED substrate is absorbed by the phosphor, and the other part of the blue light is mixed with the yellow light emitted by the phosphor to obtain white light. Now, for InGaN/YAG white LEDs, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, white light of various colors with a color temperature of 3500-10000K can be obtained.