They were used in LEDs, displays, instrument panels of cars and airplanes, advertising displays, different systems of information visualization. On light output LEDs have overtaken conventional incandescent lamps. For durability, reliability, security, they also surpassed them. One was bad – there were no blue LEDs, blue-green and white. 8. What determines the color of the led? Exclusively on the band gap, which recombine with electrons and holes, ie, from semiconductor material, and the dopants. Than the 'blue' led, the higher the photon energy, and hence the greater must be the band gap. 9. What difficulties have had to overcome a rocket scientist to make a blue led? Blue LEDs can be made on the basis of Semiconductors with wide band gap – silicon carbide, compounds of the elements ii and Group iv or Group iii nitrides. (Remember the periodic table?) Of LEDs based on SiC was too low efficiency and low quantum yield (ie the number of emitted photons to recombine a pair). In LEDs based on solid solutions of zinc selenide ZnSe quantum yield was higher, but they are overheated due to the large resistance and were short-lived. There was hope for the nitrides. Gallium nitride gn melts at 2000 C, and the equilibrium vapor pressure of nitrogen at 40 atmospheres, it is clear that such crystals are difficult to grow. Similar compounds – aluminum and nitriles India – are also semiconductors. These connections form ternary alloys with a band gap, which depends on the composition, which can be chosen so as to generate a desired light wavelength, including blue.