A very important light source in daily life is LED(Light emitting diode). A light emitting diode is an important light source in daily life. We can easily be seen it in television remote control. It used in optical communication. Modern time LED’s producing visible light are used in a numerical display like watches at the railway platform, calculators, instrument panels, telephone switchboards etc.
LED is a monochromatic light source. It is a p-n junction diode made of gallium phosphide or gallium arsenic phosphide. P-N junction diode is a semiconductor device which is very useful in all electronic types of equipment. Basic important of semiconductor arises due to its conductivity, which lies between conductor and insulators. LED works when p-n junction diode is forward biased. Electron and holes present in diode move towards the junction in opposite direction. As they cross the junction, the electrons and holes recombine and energy is released in the form of light. We can choose a different type of semiconductor material for LED and by a proper choice of semiconducting material, we can obtain any colour light. Blue colour LED is the latest invention and Noble Prize announce 2014 for this discovery.
LED manufactured using gallium arsenide produce red light and LED manufactured using gallium phosphide produce green light. The basic theory behind its working can explain on the basis of band theory of semiconductors solids. The LED voltage rating of most LED is 1V to 2.5 V and current around 20 mA to 100mA. So its a low voltage operating device. It’s working (on-off switching)very fast and it has very long lifetime approximately 20 years. LED can bear maximum 3 V in reverse biased condition if the reverse voltage applied to LED is more than 3V it may be destroyed. To protect LED this situation a rectifier diode connect to LED which protects the LED from damage.