Q. Explain the working of LEDs

OPTOELECTRONIC devices either produce light or use light in their operation. The first of these, the light-emitting diode (LED), was developed to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on panels. A LIGHT-EMITTING DIODE is a diode which, when forward biased, produces visible light. The light may be red, green, or amber, depending upon the material used to make the diode. Figure shows an LED and its schematic symbol. The LED is designated by a standard diode symbol with two arrows pointing away from the cathode. The arrows indicate light leaving the diode. The circuit symbols for all optoelectronic devices have arrows pointing either toward them, if they use light, or away from them, if they produce light. The LED operating voltage is small, about 1.6 volts forward bias and generally about 10 mill amperes. The life expectancy of the LED is very long, over 100,000 hours of operation.

The atomic structure of the LED is carefully designed so that as free electrons cross the junction from the N-type side to the P-type side, the amount of energy each electron releases as it drops into a nearby hole corresponds to the energy of a photon of some particular color. Therefore, that photon is released as a visible photon of that color. Gallium Arsenide (GaAs) crystal has the interesting property of radiating significant amounts of infrared radiation from the junction. By adding Phosphorus to the equation, they shortened the wavelength of the emitted radiation until it became visible red light. Further refinements have given yellow and green LEDs. More recently, blue LEDs have been produced, by putting nitrogen into the crystal structure. This makes full-color flat-screen LED displays possible.

LEDs are used widely as "power on" indicators of current and as displays for pocket calculators, digital voltmeters, frequency counters, etc. For use in calculators and similar devices, LEDs are typically placed together in seven-segment displays.