LED are light emitting diodes. It works in just the opposite way a photo diode works. Photodiodes receive light and hence conducts differently. But LEDs emit light using electric current. It is again a P-N junction made of gallium - arsenate or indium phosphate. While a photodiode works under reverse bias condition, LEDs work under forward bias condition. When a conduction electron makes a transition to valence band to fill up a hole in a PN junction, the extra energy is emitted as a photon. If the wavelength of the emitted photon happens to lie in the visible part of the electromagnetic spectrum, we can see the emitted light. LED's are used as indicator light in burglar alarms, calculators, digital watches etc.
