LED (Light Emitting Diode )

LED (Light Emitting Diode) :-  A PN junction , which emits the light when forward biased ,is known as a light emitting diode 

-It just like a P N junction photodiode that can absorb light and produce an electrical current , the opposite is also possible that a pN Junction can also emit light or exhibit electroluminescence. Each recombination of ele elctron -hole pair radiates energy .

-in an ordinary diode , the radiated energy is in the form of electromagnetic heat because germanium and silicon diode have less probabilities of radiating light.

constructional Diagram :- 

• LED's are fabricated by using vapour phase or liquid phase epitaxy.
• Direct band gap red LED's are fabricated on GaAs substrate  and indirect band gap LED's are an GaP substrate . indirect band gap LED's emit orange,yellow or green light.
• The N-type layer is grown on a p-type substate by diffusion process as shown in fig A , then a thin P type layer is grow on N type layer 
• The metal connection to both the layer make anode and cathode terminals as indicated. the transparent windows is provided at the top of the surface.
• the germanium and silicon semiconducting materials are not used  for manufacturing LED's because these are heat producting materials, moreover ,these materials are very poor in emitting light radiations.

figure A. vapour phase epitaxy structure

                                                    figure b .liquid phase epitaxy structure

Material used for LED :-

• the semiconducting materials used for manufacturing visible LED's are gallium phosphide (GaP), gallium arsenide phosphide (GaAsP) and gallium nitride (GaN).
• the visible LED's radiate light in different colours such as red, green, yellow, blue, orange etc.
• the GaAsP emits either red or yellow light . GaP emits red or green light and GaN emits Blue light.
• the best semiconductor material used for manufacturing infrared(IR) LEDs is gallium Arsenide (GaAs)

Symbol:-

figure C : lamp construction and symbol

Advantages :-

1. It has extremely high operating speed (few nanoseconds) of operation.
2. it is cheap i.e  of low cost and hence economical .
3. it can be easily interfaced with other electronics circuits.
4. it has low inherent noise.
5. it has low voltage operation, making them compatible with IC's
6. it is available in variety of colours.
7. it has high immunity to external noise.

disadvantages:-

1. the radient output power and it's wavelength are temperature dependent . hence ,these are effected by changes in temperature.
2. it is liable to get damaged by over voltage or over current.
3. it needs larger power for its operation.
4. theoretically overall efficiency is not achieved except in special cooled or pushed conditions, hence its luminous efficiency is low. 

Applications:-

• it is used in optocouplers.
• used in optical switching applications
• used in optical communication systems
• used in solid states video playback displays and CRT.
• it is used in remote controllers (IR - Infrared led's)
• it is used in indicators for different applications.
• it is used in digital logic gates, displays, alarm clock. 7segment displays, videogames, voltage level indicators, backlight keyboards, sign boards etc.