Describe working principle of LED with diagram.
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Diagram:

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Working: A PN junction diode, which emits light when forward biased, is known as a Light Emitting Diode (LED). The emitted light may be visible or invisible. The amount of light output is directly proportional to the forward current. Thus, higher the forward current, higher is the light output. 

When the LED is forward biased, the electrons and holes move towards the junction and the recombination takes place. After recombination, the electrons, lying in the conduction bands of N region, fall into the holes lying in the valence band of a P region. The difference of energy between the conduction band and valence band of a P region is radiated in the form of light energy. The semiconducting materials used for manufacturing of Light Emitting Diodes are Gallium Phosphide and gallium Arsenide Phosphide. These materials decide the colour of the light emitted by the diode.

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The lighting emitting diode is a p-n junction diode. It is a specially doped diode and made up of a special type of semiconducting materials like Gallium Phosphide(GaP), Gallium Arsenide Phosphide(GaAsP) and Gallium Nitride(GaN). 


The working principle of the light emitting diode is based on the quantum theory. 


The quantum theory says that when the electron comes down from the higher energy level to the lower energy level then, the energy emits from the photon. 


When the LED is forward biased, the electrons in the n-region will cross the junction and recombine with the holes in the p-type material. 


These free electrons reside in the conduction band and hence at a higher energy level than the holes in the valance band. 


When the recombination takes place, these electrons return to the valance band which is at lower energy level than the conduction band. 


While returning , the recombining electrons give away the excess energy in the form of light. 


Color of emitted light depends on which material is used to manufacture the diode.

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construction details of LED

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The construction of LED differs from normal standard diode in many aspects. As shown in the figure the p-n junction of the LED bulb is enclosed by a transparent, hard plastic epoxy resin hemispherical-shaped shell or body which protects the LED from shock and vibration.

LEDs have two terminals; a cathode and an anode. The cathode terminal is identified by either a flat spot on the body or notch or by one of the leads shorter than the other. The domed top of the LED is just like a lens concentrating the amount of light. The epoxy resin shell is constructed in such a way that the photons of light emitted by the junction are reflected away from the substrate base to which the diode is attached because LED junction does not emit that much light. Due to this, the brightest light will be emitted at the top of the LED.

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