Read the following text and answer the following questions on the basis of the same:
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 `to` p (where they are minority carriers) and holes are sent from p`to`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.
LED is a :

Options

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.

Explanation

**Step-by-Step Solution:**

1. **Identify the Definition of LED**: The text states that a Light Emitting Diode (LED) is a "heavily doped p-n junction." This is crucial information for answering the question.

2. **Examine the Options**: The options provided are:
- Option 1: Lightly doped p-n junction diode
- Option 2: Heavily doped p-n junction diode
- Option 3: Moderately doped p-n junction diode
- Option 4: Two back to back p-n junction diodes

3. **Analyze Each Option**:
- **Option 1**: Lightly doped p-n junction diode - This contradicts the text, which specifies that LEDs are heavily doped.
- **Option 2**: Heavily doped p-n junction diode - This matches the description provided in the text.
- **Option 3**: Moderately doped p-n junction diode - This also contradicts the text.
- **Option 4**: Two back to back p-n junction diodes - This is not mentioned in the text and is therefore incorrect.

4. **Select the Correct Answer**: Based on the analysis, the correct answer is **Option 2: Heavily doped p-n junction diode**.

5. **Conclusion**: The LED is defined as a heavily doped p-n junction diode that emits light when forward biased.

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