A light-emitting diode (LED) is a special type of diode that emits light when it is conducting.  Just like an ordinary diode, a light-emitting diode also has a p-n junction that allows it to conduct current much more easily in one direction than the other. As in ordinary diodes, it conducts current only when it is forward-biased, i.e., its p-side is more positive than its n-side by about 0.7 V.  The p-side of an LED is known as an anode while its n-side is known as a cathode. 

In a semiconductor device, the charge carriers that comprise the flow of current are electrons and holes. When an electron meets a hole during device operation, they annihilate each other, since a hole is basically just the absence of an electron.  The electron then goes into a lower-energy state, releasing its 'excess' energy first before doing so.  In ordinary diodes, the energy released is not visible.  In LED's, however, the energy released is in the form of visible optical emissions.  This is why an LED emits light during conduction.

LED's are often used in digital indicators and signs, in decorative applications, or even for illumination purposes.  Figure 1 shows a photo of an LED (left), a photo of a car's tail light consisting of numerous LED's (center), and the circuit symbol for an LED (right).  The LED's circuit symbol is just the symbol for a regular diode combined with arrows signifying the emission of photons.

Figure 1.  Photo of a light-emitting diode (left); a car tail light consisting of LED's (center); and the circuit symbol for an LED (right)

The wavelength of the light emitted by an LED, and therefore its color, are determined by the band gap energy of the semiconductor material used in forming the p-n junction.  The materials used for LED's are chosen to have band-gap energies that correspond to near-infrared, visible, or near-ultraviolet light, to make them emit light during operation. Table 1 shows some semiconductor materials used for LED's and their corresponding emission color.

Table 1.  Some Semiconductors Used for LED's and their Colors

LED's are often fabricated on an n-type substrate, although p-type substrates are also used for the same purpose.  Substrates that are transparent to the light emission and backed by a reflective layer, increase the efficiency of the LED.  The microchip of an LED is encapsulated in a tough, solid plastic lens. The refractive index of the LED package must be compatible with the semiconductor used; otherwise, the light emitted gets reflected back into the semiconductor where it is absorbed and dissipated as heat.

Being a diode, an LED requires correct polarity (it must be forward-biased) in order to emit light.  LED's must not be subjected to large currents, since they are easily destroyed by electrical overstress.  A resistor is often connected in series with an LED to limit the current flowing through the latter.

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