Case Study Questions for Class 12 Physics Chapter 14 Semiconductor Electronics

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Case Study Questions for Class 12 Physics Chapter 14 Semiconductor Electronics

Question 1:

Light Emitting Diode:
It is a heavily doped p-n junction which under forward bias emits spontaneous radiation. The diode is encapsulated with a transparent cover so that emitted light can come out. When the diode is forward biased, electrons are sent from n → p (where they are minority carriers) and holes are sent from p → n (where they are minority carriers). At the junction boundary, the concentration of minority carriers increases as compared to the equilibrium concentration (i.e., when there is no bias).
Thus at the junction boundary on either side of the junction, excess minority carriers are there which recombine with majority carriers near the junction. On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted. When the forward current of the diode is small, the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum. Further increase in the forward current results in decrease of light intensity. LED’s are biased such that the light emitting efficiency is maximum.
The V-I characteristics of a LED is similar to that of a Si junction diode. But, the threshold voltages are much higher and slightly different for each colour. The reverse breakdown voltages of LED’s are very low, typically around 5 V. So care should be taken that high reverse voltages do not appear across them. LED’s that can emit red, yellow, orange, green and blue light are commercially available.

Read the above case/passage and answer the following questions:

(i) LED is a:
(a) lightly doped p-n junction diode.
(b) heavily doped p-n junction diode.
(c) moderately doped p-n junction diode.
(d) two back to back p-n junction diode.

Answer Answer:
Option (b) is correct.
Explanation: LED is a heavily doped p-n junction diode.

(ii) LED emits light:
(a) when reversed biased.
(b) when forward biased.
(c) when forward or reverse biased.
(d) when heated.

Answer Answer:
Option (b) is correct.
Explanation: LED under forward bias emits spontaneous radiation.

(iii) During recombination at the junction, emitted photons have:
(
a) energy equal to or slightly less than the band gap.
(b) energy greater than the band gap.
(c) energy which has no relation with the band gap.
(d) very low energy compared to band gap.

Answer Answer:
Option (a) is correct.
Explanation: On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted.

(iv) Threshold voltage of LED is:
(a) lower compared to other p-n junction diodes and slightly different for each colour.
(b) lower compared to other p-n junction diodes and same for all colours.
(c) higher compared to other p-n junction diodes and same for all colours.
(d) higher compared to other p-n junction diodes and slightly different for each colour.

Answer Answer:
Option (d) is correct.
Explanation: The V-I characteristics of a LED is similar to that of a Si junction diode. But the threshold voltages are much higher and slightly different for each colour.

(v) The reverse breakdown voltages of LED’s are:
(a) very low and typically around 0.5 V.
(b) very high and typically around 50 V.
(c) very low and typically around 5 V.
(d) very low and typically around 0.05 V.

Answer Answer:
Option (c) is correct.
Explanation: The reverse breakdown voltages of LED’s are very low, typically around 5 V.

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