Since the discovery of the red LED (1962), the development of solid-state light sources has not stopped for a moment. Each decade was marked by scientific advances and opened new horizons for scientists. In 1993, when Japanese scientists managed to obtain blue light and then white light, the development of LEDs moved to a new level. Physicists all over the world faced a new challenge, the essence of which was to use LED lighting as the main one.
Nowadays, the first conclusions can be drawn indicating the success of the development of LED lighting and the ongoing modernization of the LED. Luminaires with LEDs manufactured using COB, COG, SMD, filament technologies appeared on store shelves.
How does each of these types work, and what physical processes force a semiconductor crystal to glow?
Device, design and technological differences
There are many signs by which LEDs can be classified into groups. One of them is the technological difference and the slight difference in the device, which is caused by the peculiarity of the electrical parameters and the future field of application of the LED.
DIP
The cylindrical, double-ended epoxy package was the first design for a light-emitting crystal. A rounded colored or transparent cylinder serves as a lens, forming a directed beam of light. The pins are inserted into holes in the printed circuit board (DIP) and soldered to provide electrical contact.
The emitting crystal is located on the cathode, which has the shape of a flag, and is connected to the anode by the thinnest wire. There are models with two and three crystals of different colors in one package with the number of pins from two to four. In addition, a microchip can be built inside the case, which controls the sequence of the crystals glowing or sets the purity of its blinking. 
LEDs in a DIP package are low-current ones, used in backlighting, display systems and garlands.
In an attempt to increase the luminous flux, an analogue with an improved device in a DIP package with four pins, known as "piranha", appeared. However, the increased light output was offset by the size of the LED and strong heating of the crystal, which limited the scope of the "piranha". And with the advent of SMD technology, their production has practically ceased.
SMD
Surface-mount semiconductors are fundamentally different from their predecessors. Their appearance expanded the possibilities of designing lighting systems, made it possible to reduce the dimensions of the luminaire and fully automate the installation. Today SMD-LED is the most demanded component used for building light sources of any format.
The base of the case, on which the crystal is attached, is a good heat conductor, which significantly improved heat dissipation from the light-emitting crystal. In the device of white LEDs between the semiconductor and the lens, there is a layer of phosphor to set the desired color temperature and neutralize ultraviolet radiation. In SMD components with a wide angle of radiation, there is no lens, and the LED itself has the shape of a parallelepiped.
COB
Chip-On-Board is one of the newest practical advances that will take the lead in the production of white LEDs in artificial lighting in the near future. A distinctive feature of the LED device is as follows: tens of crystals without a housing and a substrate are attached to an aluminum base (substrate) through a dielectric glue, and then the resulting matrix is covered with a common phosphor layer. The result is a light source with an even distribution of the luminous flux, eliminating the appearance of shadows.
A variation of COB is Chip-On-Glass (COG), which involves placing many small crystals on a glass surface. In particular, it is widely known in which the emitting element is a glass rod with LEDs coated with a phosphor.
LED working principle
Despite the considered technological features, the operation of all LEDs is based on the general principle of operation of the emitting element. The conversion of electric current into a luminous flux occurs in a crystal, which consists of semiconductors with different types of conductivity. A material with n-conductivity is obtained by doping it with electrons, and a material with p-conductivity with holes. Thus, additional charge carriers of the opposite direction are created in adjacent layers. 
At the moment the forward voltage is applied, electrons and holes begin to move to the pn junction. The charged particles overcome the barrier and begin to recombine, resulting in an electric current flowing. The process of recombination of a hole and an electron in the p-n-junction zone is accompanied by the release of energy in the form of a photon.
In general, this physical phenomenon applies to all semiconductor diodes. But in most cases, the photon's wavelength is outside the visible radiation spectrum. To make an elementary particle move in the range of 400-700 nm, scientists had to carry out many experiments with the selection of suitable chemical elements. As a result, new compounds appeared: gallium arsenide, gallium phosphide and their more complex forms, each of which is characterized by its own wavelength, and hence the color of the radiation.
In addition to the useful light emitted by the LED, some heat is released at the pn junction, which reduces the efficiency of the semiconductor device. Therefore, in the design of high-power LEDs, the possibility of implementing effective heat dissipation should be considered.
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