CBSE 2025, Set 1

Question

**Assertion (A):** The impurities in p-type Si are not pentavalent atoms.

**Reason (R):** The hole density in valence band in p-type semiconductor is almost equal to the acceptor density. A). Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). B). Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). C). Assertion (A) is true, but Reason (R) is false. D). Both Assertion (A) and Reason (R) are false.

Edvaya · Accepted Answer

The correct answer is Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).. **Assertion (A) — TRUE:** p-type semiconductors are made by doping Si (Group IV) with **trivalent** (Group III) impurities such as Boron (B), Gallium (Ga), or Indium (In). Pentavalent atoms (like P, As, Sb) are used to make **n-type** semiconductors. So A is correct. **Reason (R) — TRUE:** In a p-type semiconductor, each acceptor atom (trivalent impurity) accepts one electron from the valence band, creating one hole. Therefore: $$n_h \approx N_A 	ext{ (acceptor density)}$$ This is a well-established fact in semiconductor physics. R is correct. **Why R does NOT explain A:** Assertion A is about **which atoms** are used (trivalent, not pentavalent). Reason R describes the **resulting hole density** in p-type material. R is a consequence of doping with trivalent impurities, but does not explain *why* pentavalent atoms are not used. The explanation for A would be: pentavalent atoms donate electrons (making n-type), not holes. **Answer: (B) — Both true, R is not the correct explanation of A.** > **Key fact:** p-type doping uses Group-III elements (B, Al, Ga, In); n-type uses Group-V elements (P, As, Sb). This is determined by valence electron count relative to Silicon's 4.

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