Assertion: Thickness of depletion layer is fixed in all semiconductor devices.
Reason: No free charge carriers are available in deplection layer.

Explanation

To analyze the given assertion and reason, we will break down the concepts involved in the depletion layer of a p-n junction diode.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "the thickness of the depletion layer is fixed in all semiconductor devices."
- This is not true because the thickness of the depletion layer can vary depending on the biasing conditions of the semiconductor device.

**Hint**: Consider how external factors like voltage can influence the depletion layer.

2. **Understanding the Reason**:
- The reason states that "no free charge carriers are available in the depletion layer."
- This statement is true. In the depletion layer, the majority carriers (holes in p-type and electrons in n-type) recombine, leaving behind a region devoid of free charge carriers.

**Hint**: Recall what happens to charge carriers at the junction of p-type and n-type materials.

3. **Analyzing the p-n Junction**:
- In a p-n junction, when the p-type and n-type materials are brought together, electrons from the n-side recombine with holes from the p-side, creating a depletion region.
- This region is characterized by the absence of free charge carriers, leading to the formation of an electric field.

**Hint**: Think about the movement of electrons and holes at the junction.

4. **Effect of Biasing on Depletion Layer**:
- When the p-n junction is forward-biased, the external voltage reduces the width of the depletion layer because it provides energy to the charge carriers, allowing them to recombine more easily.
- Conversely, in reverse bias, the depletion layer widens as the external voltage opposes the built-in electric field, pulling the charge carriers away from the junction.

**Hint**: Consider how applying voltage in different directions affects the behavior of the depletion layer.

5. **Conclusion**:
- Since the thickness of the depletion layer is not fixed and varies with the biasing conditions, the assertion is false.
- However, the reason that there are no free charge carriers in the depletion layer is true.

### Final Answer:
- The assertion is **false** and the reason is **true**. Therefore, the correct conclusion is that the assertion does not hold while the reason is valid.