Question

When a small amount of impurity atoms are added to a semiconductor then generally its resistivity

• A. increases
• B. decreases
• C. does not change
• D. may increase or decrease depending upon the percentage of doping

Hint:

Impurity atoms add charge carriers, so conductivity rises and resistivity falls.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A pure (intrinsic) semiconductor has few free charge carriers at room temperature. Doping adds donor or acceptor atoms.

Step 2: Detailed Explanation:
A pentavalent impurity adds free electrons (n-type). A trivalent impurity adds holes (p-type). In both cases the number of charge carriers goes up by a large factor, even for a very small doping fraction.
Conductivity \(\sigma = e(n\mu_e + p\mu_h)\) rises, so resistivity \(\rho = 1/\sigma\) falls.

Step 3: Final Answer:
The resistivity generally decreases, option (B).

Final Answer:
Doping adds carriers, so resistivity falls. \[ \boxed{\text{(B) }\text{decreases}} \]