VKC ScienceChemistry for written exams Telegram

UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper II

Q1(d). Intrinsic semiconductor and its conversion to extrinsic

  • 5 marks
  • Topic: Solid state
  • Answered
Question

Which one of the following is an intrinsic semiconductor and how do you convert it into an extrinsic semiconductor? (i) diamond (ii) α-tin (iii) silicon (iv) YBa₂Cu₃O₇

Answer in brief

Silicon (band gap 1.1 eV). It is made extrinsic by doping: group 15 atoms (P, As) give n-type; group 13 atoms (B, Ga) give p-type.

Key points

  • (i) Diamond: band gap 5.5 eV, an insulator. (ii) α-tin (grey tin): gap almost zero (0.08 eV), semimetal. (iii) Silicon: gap 1.1 eV, pure Si is an intrinsic semiconductor. (iv) YBa₂Cu₃O₇: a superconductor below 92 K.
  • dope with 10⁻⁶ fraction of P: the fifth electron sits in a donor level just below the conduction band. p-type: dope with B: a hole level just above the valence band (acceptor).

The full model answer

Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.

  • (i) Diamond: band gap 5.5 eV, an insulator. (ii) α-tin (grey tin): gap almost zero (0.08 eV), semimetal. (iii) Silicon: gap 1.1 eV, pure Si is an intrinsic semiconductor. (iv) YBa₂Cu₃O₇: a superconductor below 92 K.
  • n-type: dope with 10⁻⁶ fraction of P: the fifth electron sits in a donor level just below the conduction band. p-type: dope with B: a hole level just above the valence band (acceptor).
Model answer to Q1(d), 2026 Mains Paper II: solution page 4
Solution page 4 of 22
Read the whole 2026 solution2026 question paper

CoverssemiconductorsIntrinsic semiconductor and its conversion to extrinsic

All parts of Question 1