Draw symbol of LED and explain it’s working principle.
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Working Principle of LED:

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When the LED is forward biased, the electrons in the n-region will cross the junction and re-combine 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 t h e recombination takes place, these electrons return back to the valance band which is at lower energy level than the conduction band. While returning back, the recombining electrons give away the excess energy in the form of light. This process is called as ―electroluminescence‖. In this way an LED emits light.

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Construction of LED: The basic structure of LED is shown in fig.(a). The active region exists between the p and n regions. The light emerges from the active side in all the directions when electron hole-pairs recombine. The disadvantage of this structure is that the LED emits light in all the directions. This problem is solved by placing the basic structure inside a small reflective cup so as to focus the light in the desired direction. Such a structure is called as a cup type construction and is shown in fig.(b).

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Working Principle of LED:

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See fig(c) ,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 b a n d . When t h e recombination takes place, these electrons return back to the valance band which is at lower energy level than the conduction band. While returning back, the recombining electrons give away the excess energy in the form of light. This process is called as “electroluminescence”, shown in fig.(c). In this way an LED emits light. This is the principle of operation of LED.

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