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प्रश्न
With what considerations in view, a photodiode is fabricated? State its working with the help of a suitable diagram.
Even though the current in the forward bias is known to be more than in the reverse bias, yet the photodiode works in reverse bias. What is the reason?
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उत्तर
The photodiode is special-purpose silicon diode. It is fabricated with a transparent window to expose its junction to light radiations. It always works on reverse bias condition below the breakdown voltage
When visible light of energy (hv > Eg) enters its depletion region, the electron-hole pairs are generated. These charge carriers are separated by the junction's electric field and are made to flow across the junction and cause reverse saturation current. The value of reverse saturation current depends on the intensity of incident radiation and is independent of reverse bias
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The photodiode is operated in reverse bias condition because in the reverse bias condition the change in reverse saturation current is directly proportional to the change in incident light intensity. Thus, the photodiode can be used to detect the optical signals. It cannot be done if the photodiode is forward biased.
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संबंधित प्रश्न
If a small voltage is applied to a p-n junction diode, how will the barrier potential be affected when it is(ii) reveres biased?
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Write briefly the important processes that occur during the formation of p−n junction. With the help of necessary diagrams, explain the term 'barrier potential'.
Draw V − I characteristics of a p-n junction diode. Answer the following questions, giving reasons:
(i) Why is the current under reverse bias almost independent of the applied potential up to a critical voltage?
(ii) Why does the reverse current show a sudden increase at the critical voltage?
Name any semiconductor device which operates under the reverse bias in the breakdown region.
Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode in reverse bias. Plot the V-I characteristics in this case.
With reference to Semiconductor Physics,
Name the process that causes depletion region in a p-n junction.
A p-n photodiode is fabricated from a semiconductor with a band gap of 2.5 eV. lt can detect a signal of wavelength ______.
Consider the following statements (A) and (B) and identify the correct answer.
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Read the following paragraph and answer the questions.
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LED is a heavily doped P-N junction which under forward bias emits spontaneous radiation. When it is forward-biased, due to recombination of holes and electrons at the junction, energy is released in the form of photons. In the case of Si and Ge diode, the energy released in recombination lies in the infrared region. LEDs that can emit red, yellow, orange, green and blue light are commercially available. The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV. The compound semiconductor Gallium Arsenide – Phosphide is used for making LEDs of different colours.
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- Why are LEDs made of compound semiconductor and not of elemental semiconductors?
- What should be the order of bandgap of an LED, if it is required to emit light in the visible range?
- A student connects the blue coloured LED as shown in the figure. The LED did not glow when switch S is closed. Explain why?
OR
iii. Draw V-I characteristic of a p-n junction diode in
(i) forward bias and (ii) reverse bias
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