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Draw the energy band… (Class 12 Physics SQP 2024-25, Q25I)

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PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2024-25: question 25(I)

From the official CBSE sample question paper for 2024-25, section C, question 25(I), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25(I) · 3 marks · Short answerSemiconductor Electronics: Materials, Devices and Simple Circuits

Draw the energy band diagram for P-type semiconductor at (i) T=0K and (ii) room temperature.
(II) In the given diagram considering an ideal diode, in which condition will the bulb glow
(a) when the switch is open
(b) when the switch is closed
Justify your answer.

Figure in the paper: Circuit: a cell in series with a bulb; a branch consisting of a switch in series with a diode is connected in parallel with the bulb. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

In a p-type semiconductor the impurity level lies just above the valence band.

Hint 2

An ideal diode in forward bias behaves like a plain wire. What happens to a bulb with a wire across it?

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. (I)(i) T = 0 K: the valence band is completely filled and the conduction band is completely empty, separated by the energy gap . Acceptor energy level is a short dashed line just above the top of the valence band (≈ 0.01–0.05 eV); it is empty.

    1 mark

  2. (ii) Room temperature: electrons from the valence band have jumped into the acceptor level, so is shown filled and many holes appear in the valence band (majority carriers). A few electrons have crossed into the conduction band by thermal excitation (minority carriers).

    1 mark

  3. (II) The bulb glows in case (a), when the switch is open. The diode branch is then broken, so the full current of the cell passes through the bulb.

    ½ mark

  4. When the switch is closed, the ideal diode is forward biased and has zero resistance. It short-circuits the bulb, so the current by-passes the bulb and it does not glow.

    ½ mark

Answer: (I) At 0 K: empty acceptor level just above a full valence band, empty conduction band; at room temperature: acceptor level filled, holes in valence band, few electrons in conduction band. (II) Bulb glows when the switch is open (a).

Where the marks are

  • Band diagram at 0 K with empty acceptor level near the valence band (1 mark)
  • Band diagram at room temperature with filled acceptor level, holes in VB and few electrons in CB (1 mark)
  • Bulb glows when the switch is open (½ mark)
  • Justification: closed switch puts a forward-biased ideal diode (zero resistance) across the bulb, shorting it (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 paperSample paper 2024-25: 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.