Q18 of 26 Page 1

How is a light emitting diode fabricated? Briefly state it’s working. Write any two important advantages of LEDs over the conventional incandescent low power lamps.

A light emitting diode is formed by heavily doping both n and p regions of a junction diode. The entire structure is then placed in a transparent casing for the light to be emitted after recombination.


Working of LED:


A LED works in forward biased configuration. After applying the forward bias, the depletion region or the potential barrier decreases. The electrons and holes combine in this depletion region giving off photons of energy equal to the bandgap () of the semiconductor.



LED offers several advantages over incandescent lamps:


(i)LED has no filament like features and hence have a longer lifetime than filament bulbs.


(ii)LED works on low operational voltage thus power consumption is less.


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Explain, with the help of suitable diagram, the two important processes that occur during the formation of p-n junction. Hence define the terms: depletion region and barrier potential.

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Draw a block diagram of a simple modulator for obtaining amplitude modulated signal.

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(a) The light from a clear blue portion of the sky shows a rise and fall of intensity when viewed through a polaroid which is rotated. Describe, with the help of a suitable diagram, the basic phenomenon/process which occurs to explain this observation.

(b) Show how light reflected from a transparent medium gets polarized. Hence deduce Brewster’s law.