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Compare the energy… (Class 12 Physics SQP 2026-27, Q22)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2026-27: question 22

From the official CBSE sample question paper for 2026-27, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerSemiconductor Electronics: Materials, Devices and Simple Circuits

Compare the energy band diagrams of metals, insulators, and semiconductors.

I. Explain how the band gap determines conductivity.
II. Apply this reasoning to justify why diamond (C) is an insulator while tin (Sn) is a conductor, though both are the elements of group IV.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Focus on the gap between the valence and conduction bands in each case.

Hint 2

Compare the room-temperature thermal energy (~0.025 eV) with each gap.

Hint 3

Going down group IV, the gap shrinks from C to Si to Ge to Sn.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Band diagrams: Metals – conduction band partly filled or overlapping the valence band (no gap). Insulators – full valence band and empty conduction band separated by a large gap ( eV). Semiconductors – similar but with a small gap ( eV, e.g. Si 1.1 eV).

    1 mark

  2. I. Conductivity needs electrons in the conduction band. With no gap (metal) many free electrons exist → high conductivity. With a large gap, thermal energy at room temperature ( eV) cannot excite electrons across → negligible conductivity. With a small gap, some electrons are thermally excited, leaving holes → moderate conductivity that rises with temperature and doping.

    1 mark

  3. II. Diamond (C) has a large gap ( eV), so practically no electrons reach the conduction band at room temperature → insulator. In tin (Sn), the bands overlap (zero gap), so free electrons are always available → conductor. Thus, though both are group IV, the size of the band gap decides conductivity.

    1 mark

Answer: Metals: overlapping bands; semiconductors: small gap; insulators: large gap. Diamond (Eg ≈ 5.4 eV) is an insulator; tin (overlapping bands, Eg ≈ 0) is a conductor.

Where the marks are

  • Correct comparison of band diagrams of the three classes (1 mark)
  • Explains how band gap controls number of carriers and conductivity (1 mark)
  • Diamond large gap → insulator; tin overlapping bands → conductor (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Semiconductor Electronics: Materials, Devices and Simple Circuits, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.