Even in 1907, it was first noted a faint glow emitted by silicon carbide crystals due to the unknown time electronic transformations. In 1923 O.V.Losev noted this phenomenon during the research it conducts radio with semiconductor detectors, but the intensity of the observed radiation was so small, that the scientific community for the time being is not seriously interested in this phenomenon. Only in 1962, a group of engineers led by Henry Holonyak of General Electric demonstrated its first LED, and six years later came the red LEDs on the market.
The principle of operation of the LED.
Luminescence occurs when the recombination of electrons and holes in the pn-junction. So, first of all, we needed pn-junction, that is contact between two semiconductors with different types of conductivity. To do this, the contact layers of semiconductor crystal doped with different impurities: on one side of the acceptor, on the other – the donor.
But not everyone pn-transition emits light. Why is that? Firstly, the gap width in the active region of the LED must be close to the photon energy of the visible light range. Second, the emission probability in the recombination of electron-hole pairs must be high, for which the semiconductor chip should contain few defects due to recombination which occurs without radiation. These conditions are more or less contradictory.
Really, to comply with the two conditions, a pn-junction in the crystal is not enough, and we have to produce multi-layer semiconductor structure, the so-called hetero structure for the study of which is the Russian physicist Zhores Alferov won the Nobel Prize in 2000.
The greater the current passing through the LED, the more electrons and holes come in a recombination zone per unit time. But the current can not be increased indefinitely. Because of the internal resistance of the semiconductor and pn-junction diode overheat and fail.
LED color depends solely on the band gap, wherein D combine electrons and holes, that is, from the semiconductor material, and by doping impurities. The “blue” LED, the higher the energy of the photons, and thus the greater the bandgap.
Blue LEDs can be made based on semiconductors with a large band gap – silicon carbide, compounds of the elements II and Group IV or Group III nitride (See. The periodic table).
First, back in the ’70s, the blue LED based on gallium nitride films on sapphire substrate managed to get Professor Jacques Pankov (Jacob Isayevich Panchechnikovu) from the company IBM (US). The quantum yield was sufficient for practical applications, but the leadership did not support the work Pankov.
The advantages of LEDs.
In LEDs, unlike incandescent lamps or fluorescent lamps, electric current is directly converted into light, and it is theoretically possible to make almost lossless. Indeed, LED (with proper heat removal) is heated a little, which makes it ideal for some applications. Further, the LED emits in a narrow part of the spectrum, especially the color value designers, and UV and IR radiation, as a rule, not available. The LED is mechanically strong and extremely reliable, its service life can reach 100 thousand hours, which is almost 100 times more than incandescent bulbs, and is 5 – 10 times greater than that of fluorescent lamps. Finally, LED – low voltage electrical appliances, and therefore, safe.