Which one of the following statement is not correct in the case of light emitting diodes?

Options

A. It is a heavily doped `p-n` junction
B. It emits light only when it is forwatd biased
C. It emits light only when it is reverse biased
D. the energy of the light emitted is less than the energy gap of the semicoonductor used

Explanation

To determine which statement about light-emitting diodes (LEDs) is not correct, we can analyze each option provided.

### Step-by-Step Solution:

1. **Understanding LEDs**:
- An LED is a type of diode that emits light when an electric current passes through it. It is a PN junction that is heavily doped.

2. **Forward Bias Condition**:
- LEDs operate in forward bias. This means that when the positive terminal of the power supply is connected to the anode and the negative terminal to the cathode, the diode allows current to flow.

3. **Electron Transition**:
- When the LED is forward biased, electrons from the conduction band recombine with holes in the valence band. This recombination releases energy in the form of light.

4. **Analyzing the Statements**:
- **Option 1**: "Heavily doped PN junction" - This statement is correct. LEDs are indeed heavily doped to allow for efficient light emission.
- **Option 2**: "It emits light only when it is forward biased" - This statement is also correct. LEDs emit light when forward biased.
- **Option 3**: "It emits light only when it is reverse biased" - This statement is incorrect. LEDs do not emit light when reverse biased; they only emit light in forward bias.
- **Option 4**: "The energy of light emitted is less than the energy gap of the semiconductor used" - This statement is correct. The energy of the emitted light corresponds to the energy difference between the conduction band and the valence band, which is less than the band gap energy.

5. **Conclusion**:
- The statement that is not correct regarding light-emitting diodes is **Option 3**: "It emits light only when it is reverse biased."

### Final Answer:
The incorrect statement about light-emitting diodes is Option 3: "It emits light only when it is reverse biased."

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