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Draw energy band diagrams of… (Class 12 Physics 2025, Q20)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2025: question 20

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerSemiconductor Electronics: Materials, Devices and Simple Circuits

Draw energy band diagrams of n-type and p-type semiconductors at temperature T > 0 K. Show the donor/acceptor energy levels with the order of difference of their energies from the bands.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 55/7/1.

Hints

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

Hint 1

Start with conduction and valence bands separated by about 1 eV.

Hint 2

Donor levels lie very close to one band and acceptor levels very close to the other; the gap is a few hundredths of an eV.

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. n-type diagram: draw the conduction band (top) and valence band (bottom), separated by the band gap eV. A few electrons are shown in the conduction band and a few holes in the valence band at T > 0 K.

    ½ mark

  2. Mark the **donor energy level ** as a dashed line just below the bottom of the conduction band, at eV (Ge) to eV (Si). Electrons are shown leaving this level for the conduction band.

    ½ mark

  3. p-type diagram: draw the same two bands with gap eV. Show many holes in the valence band and a few electrons in the conduction band.

    ½ mark

  4. Mark the **acceptor energy level ** as a dashed line just above the top of the valence band, at – eV. Electrons are shown jumping from the valence band to this level, leaving holes behind.

    ½ mark

Answer: n-type: donor level ~0.01–0.05 eV below the conduction band. p-type: acceptor level ~0.01–0.05 eV above the valence band. Band gap ~1 eV.

Where the marks are

  • Energy band diagram of an n-type semiconductor (½ mark)
  • Energy band diagram of a p-type semiconductor (½ mark)
  • Donor level shown just below E_C with ~0.01–0.05 eV gap (½ mark)
  • Acceptor level shown just above E_V with ~0.01–0.05 eV gap (½ 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 paper2025 board paper, set 55/7/1: 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.