What is a Light Emitting Diode

Light Emitting Diodes are one of the best opto-electronic devices present in the modern age. These are used in many applications today ranging from LED televisions to high matrix numeric and alphanumeric displays. These are also used as visual indicators in many systems.

LED are P-N junction diodes that are made up of materials like Gallium, Arsenic and Phosphorus. On applying electrical power, these diodes emit a narrow band width of either visible (red, green, amber, yellow etc.) or infrared radiations. The frequency of the radiations here is controlled by the proportions of the aforementioned materials that are used in the LED.

The action of an LED is very simple to explain. When forward bias is applied to the P-N junction of the LED, electrons from N-side cross over to the P-side and they recombine with holes there. In this recombination, the electrons jump from a higher energy state to a lower one and therefore they emit energy. While in ordinary silicon or germanium diodes the energy emitted is in the form of heat, in LED’s this energy is in the form of light. The material of the LED’s is made translucent so that this light is emitted out to the surroundings.

An LED is made by growing a P-layer over an epitaxial N-layer. Metallic contacts are provided on the surface of both layers, but on the P-region these contacts are provided near the corners to keep a vacant path for the light to escape. LED’s are also provided with domed shaped lenses to solve the problem of re-absorption of light in the material itself. Since the recombination of electrons and holes occur in the P-region, it is always kept on top of the N-region.

Notice that an LED is only used in forward biased conditions and is quickly damaged if reverse bias is applied to it. Also once the input voltage is increased beyond the knee voltage of the LED, a very large current flows through it that can damage the LED. Therefore a series limiting resistor should always be used with an LED to limit this current under the threshold values.  The brightness of the LED can also be controlled by varying this resistance accordingly.

One of the latest advent in the LED technology is the invention of multi colored LED’s whose optical outputs can be controlled by varying the magnitude of the forward voltage applied to them. The output can be controlled in frequency as well as intensity. This same technology is being used in the modern age LED televisions.

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