Which of the following diodes is used in unbiased condition ?

Options

A. Zener diode
B. Photo diode
C. Solar cell
D. LED

Answer

To determine which diode is used in an unbiased condition, we need to analyze the options provided:

1. **Zener Diode**:
- Zener diodes are typically used in reverse bias conditions. They require a specific reverse voltage to operate effectively, which means they are not used in an unbiased condition.

2. **Photodiode**:
- Photodiodes generate a current when exposed to light. They create a potential difference and require light to function, indicating that they are not used in an unbiased condition.

3. **Solar Cell**:
- Solar cells convert light energy into electrical energy. They generate current from sunlight and do not require an external voltage source to operate. This means they can function in an unbiased condition.

4. **LED (Light Emitting Diode)**:
- LEDs require a forward bias to emit light. They need an external voltage to operate, so they are not used in an unbiased condition.

Based on this analysis, the diode that can be used in an unbiased condition is the **solar cell**.

### Step-by-Step Solution:
1. **Understand the unbiased condition**: An unbiased condition means that there is no external voltage applied to the diode.
2. **Evaluate the Zener diode**: Recognize that Zener diodes operate in reverse bias and require a specific voltage, thus they cannot be used in an unbiased condition.
3. **Evaluate the photodiode**: Understand that photodiodes generate current when exposed to light and do not operate in an unbiased condition.
4. **Evaluate the solar cell**: Identify that solar cells convert light energy into electrical energy and do not require an external voltage source, making them suitable for unbiased operation.
5. **Evaluate the LED**: Acknowledge that LEDs require a forward bias to emit light and therefore cannot operate in an unbiased condition.
6. **Conclusion**: The only diode that can operate in an unbiased condition is the solar cell.

### Final Answer:
The diode used in an unbiased condition is the **solar cell**.