Assertion : Conductivity of silicon increases by doping it with
group-15 elements.
Reason : Doping means introduction of small amount of
impurities like P, As or Bi into the pure crystal.

Options

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

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that "Conductivity of silicon increases by doping it with group-15 elements."

- **Explanation**: Silicon (Si) is a semiconductor. When we dope silicon with group-15 elements such as phosphorus (P), arsenic (As), or bismuth (Bi), these elements have five valence electrons. In the silicon crystal lattice, each silicon atom is covalently bonded to four neighboring silicon atoms. When a group-15 atom replaces a silicon atom, it forms four covalent bonds with the neighboring silicon atoms using four of its five valence electrons. This leaves one extra electron that is free to move and contribute to electrical conductivity.

**Conclusion**: The assertion is true.

### Step 2: Analyze the Reason
The reason states that "Doping means the introduction of small amounts of impurities like P, As, or Bi into the pure crystal."

- **Explanation**: Doping indeed refers to the process of adding a small amount of impurities to a semiconductor to change its electrical properties. In this case, introducing group-15 elements (P, As, Bi) into silicon is a common method to enhance its conductivity by providing extra charge carriers (electrons).

**Conclusion**: The reason is also true.

### Step 3: Final Evaluation
Now we need to determine if the assertion and reason are both true and whether the reason correctly explains the assertion.

- Since both the assertion and the reason are true, we need to check if the reason correctly explains the assertion. The reason does explain how doping with group-15 elements increases conductivity, as it describes the introduction of impurities and their effect on the silicon lattice.

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