Which one of the following statement is not correct in the case of light emitting diodes?
A. It is a heavily doped p-n junction.
B. It emits light only when it is forward biased.
C. It emits light only when it is reverse biased.
D. The energy of the light emitted is equal to or slightly less than the energy gap of the semiconductor used.
Choose the correct answer from the options given below:

Solution

Physics sir · 3 steps

An LED glows only when electrons and holes meet, and they meet in forward bias, never in reverse.

1. How an LED glows. In forward bias, electrons from the n-side and holes from the p-side are pushed across the junction. When an electron drops into a hole, the energy it loses comes out as a photon.
2. Why not reverse. In reverse bias the depletion layer widens and the majority carriers are pulled away from the junction. Only a tiny current flows and almost no recombination happens, so there is no light.

   Statement	True or false	Reason
   A: heavily doped p-n junction	True	Heavy doping gives many electrons and holes, so recombination and light output are strong.
   B: emits only in forward bias	True	Forward bias injects carriers into the junction, where they recombine.
   C: emits only in reverse bias	False	Reverse bias pulls carriers away from the junction, so recombination stops and the LED does not glow.
   D: photon energy ≤ E_g	True	An electron falling across the gap releases about E_g. Some falls go through levels just inside the gap and release slightly less.

3. So. Only statement C is wrong, which is option (d).

The idea to keep. Any device that turns electron-hole recombination into light, like an LED or a laser diode, works in forward bias. The photon energy is close to the band gap E_g and never more than it.

Answer from the NTA final answer key. Solution by PrepWiser.