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Answer the following. (Class 12 Physics 2025, Q28)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/1/1 · 3 marks · Short answer

CBSE Class 12 Physics 2025: question 28

Set in the CBSE Class 12 board exam of 2025, paper 55/1/1, 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

Answer the following.

  1. (a)

    Draw circuit arrangement for studying V-I characteristics of a p-n junction diode.

    0 marks
  2. (b)

    Show the shape of the characteristics of a diode.

    0 marks
  3. (c)

    Mention two information that you can get from these characteristics.

    0 marks

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

Hints

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

Hint 1

You need two circuits: one with p connected to + and one with p connected to −. Which meter suits the very small reverse current?

Hint 2

Recall the 'knee' in the forward curve and the sudden rise in the reverse curve. What quantities can you read off at these points?

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) Forward bias circuit: a battery with a rheostat (potential divider) is connected across the diode, with the p-side to the + terminal. A milliammeter is in series with the diode and a voltmeter is across it. By varying the rheostat, readings of V and I are taken.

    ½ mark

  2. Reverse bias circuit: same arrangement with a higher-voltage battery and the n-side to the + terminal (p-side to −), using a microammeter in series and a voltmeter across the diode.

    ½ mark

  3. (b) The graph has I (mA) on the +y axis against forward V on the +x axis, and I (μA) on the −y axis against reverse V on the −x axis. Forward: current is almost zero until the threshold (knee) voltage (≈ 0.7 V for Si, 0.3 V for Ge), then rises steeply, almost exponentially. Reverse: a very small, nearly constant current (μA) flows until the breakdown voltage, where the current suddenly rises sharply.

    1 mark

  4. (c) Any two: (1) the threshold/knee voltage of the diode; (2) the reverse saturation current; (3) the breakdown voltage; (4) the diode has very low resistance in forward bias and very high resistance in reverse bias (it conducts in one direction only).

    1 mark

Answer: Forward and reverse bias circuits described; characteristic with knee at ≈0.7 V (Si) and reverse breakdown; information: knee voltage, reverse saturation current, breakdown voltage, low forward / high reverse resistance.

Where the marks are

  • Circuit for forward bias (½ mark)
  • Circuit for reverse bias (½ mark)
  • Correct shape of forward and reverse characteristics (1 mark)
  • Any two pieces of information (½ + 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 55/1/1: 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.