Light Emitting Diodes

A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. The output from an LED can range from red (at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers).

LED lighting is very different from other lighting sources such as incandescent bulbs and CFLs. Key differences include the following:

Light Source: LEDs are the size of a fleck of pepper, and a mix of red, green, and blue LEDs is typically used to make white light.
Direction: LEDs emit light in a specific direction, reducing the need for reflectors and diffusers that can trap light. This feature makes LEDs more efficient for many uses such as recessed down lights and task lighting. With other types of lighting, the light must be reflected to the desired direction and more than half of the light may never leave the fixture.
Heat: LEDs emit very little heat. In comparison, incandescent bulbs release 90% of their energy as heat and CFLs release about 80% of their energy as heat.
Safer: LEDs are much cooler than incandescent lights, reducing the risk of combustion or burnt fingers.
Longer lasting: The LEDs have very long life, almost 50 times more than ordinary bulbs, and 8-10 times that of CFLs, and therefore provide both energy and cost savings.
Easier to install: Up to 25 strings of LEDs can be connected end-to-end without overloading a wall socket.

The National Programme for LED-based Home and Street Lighting was launched to cover 100 cities by March 2016, and the remaining ones by March 2019, targeting 770 million bulbs and 35 million street lights. However, it seems street lights will be upgraded to LED ahead of schedule.

Practice Question:

1) Which of the following material are used for fabrication of LEDs?

1. Gallium Arsenide for Infra-red
2. Gallium phosphide for green light
3. Indium gallium nitride for blue light


a) Only 1

b) 1 and 2

c) 1 and 3

d) All

Ans: d
Exp: The main semiconductor materials used to manufacture LEDs are:
• Indium gallium nitride (InGaN): blue, green and ultraviolet high-brightness LEDs
• Aluminum gallium indium phosphide (AlGaInP): yellow, orange and red high-brightness LEDs
• Aluminum gallium arsenide (AlGaAs): red and infrared LEDs
• Gallium phosphide (GaP): yellow and green LEDs