Assertion: If the temperature of a semiconductor is increased then its resistance decreases.
Reason: The energy gap between conduction band and valence band is very small

Options

A. If both the assertion and reason are true and reason is a true explantion of the assertion.
B. If both the assertion and reason are true but the reason is not true the correct explantion of the assertion.
C. If the assertion is true but reason false
D. If both the assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states: "If the temperature of a semiconductor is increased, then its resistance decreases."
- In semiconductors, as temperature increases, more electrons gain enough energy to jump from the valence band to the conduction band. This results in an increase in the number of charge carriers (electrons) available for conduction.

### Step 2: Understand the Reason
The reason states: "The energy gap between the conduction band and valence band is very small."
- In semiconductors, the energy gap (band gap) is typically around 1 eV, which is relatively small compared to insulators. This small band gap allows more electrons to be thermally excited into the conduction band as the temperature rises.

### Step 3: Connect the Assertion and Reason
- As the temperature increases, the thermal energy provided to the electrons allows them to overcome the small energy gap and move into the conduction band. This results in an increase in the number of free electrons, which leads to an increase in conductivity.
- Since resistance (R) is inversely proportional to conductivity (σ), an increase in conductivity means that resistance decreases.

### Step 4: Conclusion
- Both the assertion and reason are true. The reason correctly explains why the assertion is true. Therefore, we conclude that both statements are valid.

### Final Answer
Both the assertion and the reason are true, and the reason provided explains the assertion.

---