Structure and Basic Principle of LED

Light emission diode (LED) Figure 1 shows the structural cross-section of the LED. To make LEDs emit light, the semiconductor material of the active layer must be a direct band gap material, and the electrons and holes crossing the band gap can directly compound and emit photons. In order to make the devices have good optical and carrier limitations, double heterojunction (DH) structure is mostly used.

2 basic working principle of LED

LED is a light-emitting device that directly injects current. It is the result of photons emitted by excited electrons in semiconductor crystals when they return from high energy level to low energy level, which is commonly known as spontaneous emission transition. When the PN junction of LED is applied with forward bias, the injected minority carriers and majority carriers (electrons and holes) compound and emit light. It is worth noting that for a large number of particles at high energy level, they spontaneously emit one column at an angular frequency of ν ＝ Eg / h light wave, but there is no fixed phase relationship between each column of light waves, which can have different polarization directions, and the light emitted by each particle propagates in all possible directions. This process is called spontaneous emission. The emission wavelength can be expressed by the following formula: λ ( μ m)＝1.2396/Eg(eV)

Led definition        In the PN junction of some semiconductor materials, when the injected minority carriers are combined with the majority carriers, the excess energy will be released in the form of light, so as to directly convert electrical energy into optical energy. PN junction plus reverse voltage, a few carriers are difficult to inject, so it does not emit light. This kind of diode made by the principle of injection electroluminescence is called light-emitting diode, or LED for short.        The luminous color and luminous efficiency of LED are related to the materials and technology of making LED. At present, there are three kinds of LED widely used: red, green and blue. Due to the low working voltage of LED (only 1.5-3V), it can actively emit light and has a certain brightness, the brightness can be adjusted by voltage (or current), and it is impact resistant, vibration resistant and long service life (100000 hours), there is no other display mode to compete with LED display mode in large display equipment.        The display screen made of red and green LEDs together as a pixel is called two-color screen or color screen; Red, green and blue LED tubes are put together as a pixel display screen, which is called three-color screen or full-color screen.        Whether the LED is used to make a monochrome, two-color or three-color screen, in order to display the image, the luminous brightness of each LED constituting the pixel must be adjusted, and the fineness of the adjustment is the gray level of the display screen. The higher the gray level, the finer the displayed image, the richer the color, and the more complex the corresponding display control system. Generally, for 256 gray level images, the color transition has been very soft, while for 16 gray level color images, the color transition boundary is very obvious. Therefore, color LED screens are currently required to be made of 256 gray levels.          Working principle of LED display        LED electronic display screen is a high-tech product integrating photoelectric and computer technology,        Performance characteristics of LED products        Text LED display and graphic LED display shall have the moving in and out mode and display mode specified in the detailed specification.        Computer video LED display screen shall have animation function. The animation display of LED display screen shall be consistent with that of the corresponding area of computer display; Text display function. The text display shall be stable, clear and crosstalk; Grayscale function. It is required to have the gray level specified in the detailed specification. In addition to the functions of animation, text display and gray scale, TV video LED display screen shall be able to show TV and video pictures. The market led display screen has the corresponding market display ability.        How are led displays classified        LED displays can be classified according to the following conditions:       · Use environment   LED display is divided into indoor LED display and outdoor LED display according to the use environment.       · Display color   The LED display screen is divided into single primary color LED display screen (including pseudo color LED display screen), double primary color LED display screen and full-color (three primary color) led display screen according to the display color. According to the gray level, it can be divided into 16, 32, 64, 128 and 256 gray LED displays.       · Display performance   LED display is divided into text LED display, graphic LED display, computer video LED display, TV video LED display and market led display according to display performance. Market led display screen generally includes LED display screen for securities, interest rate, futures and other purposes.       · Basic luminous point   In the non market led display screen, the indoor LED display screen can be divided into Φ 3mm、 Φ 375mm、 Φ 5mm、 Φ 8mm, and Φ 10mm display screen; The outdoor LED display screen can be divided into four types according to the adopted pixel diameter Φ 19mm、 Φ 22mm and Φ Mm26 and other LED displays. Market led displays can be divided into 2.0cm (0.8inch), 2.5cm (1.0inch), 3.0cm (1.2inch), 4.6cmm (1.8inch), 5.8cm (2.3inch), 7.6cm (3inch) and other LED displays according to the size of nixie tube used.          How to evaluate the quality of LED screen?         · Runaway point   The out of control point of indoor LED display screen shall not be greater than three ten thousandths, and the out of control point of outdoor LED display screen shall not be greater than three thousandths, and shall be discrete;       · uniformity   The consistency of LED screen shall be good, and the brightness of the whole screen shall be balanced without obvious color block;       · Security   There is no potential safety hazard and leakage current, and the leakage current to the ground shall not exceed 3.5ma (AC effective value); Resistance point strength: the LED display screen can withstand the experimental voltage lmin of 50Hz and 1500V (AC effective value), and insulation breakdown shall not occur; At the same time, the LED display screen shall have warning signs at the fuse and switching power supply;       · Appearance, no obvious scratch on the outer frame of LED display screen. The pixel tubes of outdoor LED display screen shall be installed uniformly without looseness and shell rupture;       · Structure, the LED display can be made of steel, aluminum, wood and other materials, and the structure is required to be firm and beautiful.