The light-emitting diode is based on semiconductor technology. Components used in electronics, such as diodes and transistors, are called semiconductors. Semiconductors are, however, like their name indicates, materials which conduct electricity in a weaker manner than the real conductors, such as copper.
This is caused by the fact that in the material’s atom’s outer shell composed of electrons there are four electrons in the so-called conduction band, when conduction is present it has two electrons and when it is insulating has eight. The conduction band of the insulators is thus “full,” and the electrons cannot move from one atom to another, whereas the charging materials have space for the electrons to move. The function of the semiconductors is thus somewhere between these. In electronics, the most important semiconductors are silicon and germanium
In the nucleus of the silicon atom there are 14 positively charged protons, and in its electrical field there are 14 negatively charged electrons with three shells. In the inner shell 2 electrons can be found, in the middle shell 8 and in the outer shell 4. These types of atoms can bind to one another so that they join each other in the outer shell to form a covalent bond. When the outer electron belt has four electrons, the atom can bind with four other adjacent atoms thus forming a steady crystal structure.
When small amounts of other materials, which contain more (or less) electrons in the outer shell, are added to the semiconductor, a disturbance in the covalent bond is made. This kind of disturbance causes either the gain or loss or an electron in the outermost electron shell. If the added material, for example arsenic, causes the addition of an electron (so-called donor atoms) the structure will have one electron that does not participate in the crystal structure.
Since the electron is negatively charged, this type of semiconductor is called type N. Similarly, if the semiconductor is mixed with a material, for example boron, which causes the leaking of electrons (acceptor atom), a positive charge will be generated, because one negative electron has been taken away. Then the electron that left only leaves an empty space, a hole, which acts as a positively charge carrier. This type of semiconductor is called type P.
We also talk about majority charge carriers (type N) and minority charge carriers (type P). By combining the two different types of semiconductors, we get a PN junction, which is composed of type P and type N semiconductors and the interface between them. This union forms a diode.
The density of charge carriers (holes and electrons) is the greatest in the vicinity of the diode’s interface. From the interface, electrons from the N side move to the P side, and the electrons fill available holes, which then disappear. This phenomenon is called recombination. Correspondingly, from the P side holes move to the N side and are filled with extra electrons, they recombine with the electrons.
When the charge carriers thus move across the interface, they leave behind them in the charge layer’s P side a positive charge composed of acceptor ions and in the N side of donor ions
The layer begins to resist the diffusing currents, and the diffusing currents stop. In the interface zone is thus created an area, which has donor and acceptor ions, but almost no free charge carriers.
The PN union is in balance until external energy is brought to it. If the external voltage source’s positive pole is connected to the PN union’s type N material and the negative pole to the type P material, the PN union is connected to the reverse bias. The reverse bias voltage causes the widening of the interface zone and the growth of the barrier. A slight leakage current, less than a microamper, travels to the reverse bias. This current depends on the temperature.
If an external source of voltage is connected so that the negative pole is connected to the type N material and the positive correspondingly to the type P material, the interface zone narrows and the majority charge carriers cross over the interface, and the current begins to circulate. As the voltage increases, the current’s rate rises noticeably. Nevertheless the current’s rate cannot grow unlimitedly, but it has to be regulated, for example with a resistor connected in serial. In light diodes, the current is conducted clockwise over the diode, and the recombination of the electron hole pair causes the emission of a photon (emission = the sending of particles or radiation). This causes the light effect.
Already as early as the beginning of the1920s, Henry Round noticed that the semiconductor union could generate light. In the middle of the same decade, the Russian Oleg Losev developed the first LED but went unnoticed. In 1955, Rubin Braunstein of RCA noted that the gallium-arsenide mix radiates infrared light, and in 1961 Bob Biard and Gary Pittman received the patent for IR-LED.
In 1962, Nick Holonyak developed the first LED functioning in a visible area. In 1972, George Craford invented the yellow LED and ten times as bright as the previous one red and orange LED. Professor Shuji Nakamura developed the bright blue LED based on indium gallium nitride and shortly after that by using theY3Al5O12:Ce phosphorus coating to mix yellow light with blue light he achieved the white LED. Professor Nakamura was awarded the world's biggest technology award, the Finnish Millennium Technology Prize, for his invention. The picture below shows a white light-emitting LED.
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