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Explain the following, giving… (Class 12 Physics 2024, Q28)

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PhysicsCBSE Class 12 · Board exam 2024, set 55/5/1 · 3 marks · Short answer

CBSE Class 12 Physics 2024: question 28

Set in the CBSE Class 12 board exam of 2024, paper 55/5/1, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerSemiconductor Electronics: Materials, Devices and Simple Circuits

Explain the following, giving reasons :

  1. (a)

    A doped semiconductor is electrically neutral.

    0 marks
  2. (b)

    In a p-n junction under equilibrium, there is no net current.

    0 marks
  3. (c)

    In a diode, the reverse current is practically not dependent on the applied voltage.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 55/5/1.

Hints

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

Hint 1

Doping adds neutral atoms; what is left behind when a carrier is freed?

Hint 2

Two currents flow across an unbiased junction in opposite directions.

Hint 3

Which carriers carry the reverse current, and what limits their number?

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. (a) Every extra free charge carrier contributed by a dopant atom leaves behind an ionised core of equal and opposite charge. Electrons are left behind by donor ions, and holes come with acceptor ions.

    The total positive charge therefore equals the total negative charge, so the doped semiconductor stays electrically neutral.

    1 mark

  2. (b) At equilibrium the diffusion current of majority carriers across the junction is exactly balanced by the oppositely directed drift current of minority carriers, which is driven by the barrier field.

    The two cancel, so the net current is zero.

    1 mark

  3. (c) The reverse current is carried by minority carriers, which drift across the junction under the barrier field.

    Its size is limited by the concentration of minority carriers on either side. This concentration depends on temperature, not on the applied voltage. Even a small reverse voltage sweeps all available minority carriers across, so the current saturates and hardly changes with voltage (until breakdown).

    1 mark

Answer: (a) Carrier charge balanced by ionised cores; (b) diffusion current = drift current; (c) reverse current limited by the minority carrier concentration

Where the marks are

  • Charge of the extra carriers equals and opposes that of the ionised cores (1 mark)
  • Diffusion current equals drift current at equilibrium (1 mark)
  • Reverse current limited by the minority carrier concentration, not the voltage (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 paper2024 board paper, set 55/5/1: 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.