LED Diodes

A light-emitting diode (LED) is a diode which, when forward biased, produces light. The light may be e.g. red, green, or blue, depending upon the material used to make the diode. LEDs have life spans much longer than those of incandescent bulbs and are not sensitive to shock, vibration, or extreme temperatures. LEDs are much smaller, which makes it possible for the packaging to be more rugged and weather-resistant.

Light-emitting diode technology is based on an electroluminescent phenomenon referred to as carrier injection electroluminescence. In the presence of an electric field of proper polarity, loosely bound electrons on the N-doped side of a PN junction drift (i.e., are injected) across the diode junction region where, upon entering the P-doped region as minority carriers, they combine either by direct or indirect band gap transitions with majority carrier holes producing both light and heat.

The figure below shows an LED and its schematic symbol. The LED is designated by a standard diode symbol with two arrows pointing away from the diode. The arrows indicate light leaving the diode. The circuit symbols for 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 2 volts forward bias, and the forward current is generally about 10 milliamperes.

LED.

LEDs operate in visible, infrared, and ultraviolet wavelengths. Infrared light has a wavelength greater than about 700 nm. Infrared LEDs are often used in remote-control applications. LEDs produce mostly a monochromatic color, that is, the emitted light occurs within a relatively narrow wavelength. Typically over 95% of the red LED radiant energy is emitted between 620 and 670 nm. This is within the sensitivity of the human eye, which is sensitive from 400 to 700 nm. This makes the LED a very efficient source in producing visible light.