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Explain the process of… (Class 12 Physics 2025, Q28)

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PhysicsCBSE Class 12 · Board exam 2025, set 55/7/1 · 3 marks · Short answer

CBSE Class 12 Physics 2025: question 28

Set in the CBSE Class 12 board exam of 2025, paper 55/7/1, 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 process of formation of 'depletion layer' and 'potential barrier' in a p-n junction region of a diode, with the help of a suitable diagram. Which feature of junction diode makes it suitable for its use as a rectifier ?

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

Think about what is left behind when an electron or hole diffuses across the junction.

Hint 2

Those uncovered ions create an electric field; which way does it push the carriers?

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. Depletion layer: when the p-n junction forms, electrons diffuse from the n-side (high concentration) to the p-side and holes diffuse from the p-side to the n-side. Each electron leaving the n-side leaves behind an immobile positive donor ion. Each hole leaving the p-side leaves behind an immobile negative acceptor ion. This thin region of immobile ions near the junction, with no free charge carriers, is the depletion layer. Diagram: a p-region and an n-region with a narrow central strip showing ⊖ ions on the p-side and ⊕ ions on the n-side, and an electric field pointing from n to p.

    1 mark

  2. Potential barrier: the space charge sets up an electric field from n to p, which drives a drift current opposite to the diffusion current. As diffusion continues, the layer widens and the field grows until drift current equals diffusion current, giving equilibrium. The potential difference across the layer then opposes further movement of electrons from n to p (and holes from p to n). This is called the potential barrier ( V for Si, 0.3 V for Ge).

    1 mark

  3. Rectifier feature: a junction diode conducts easily (low resistance) only when forward biased and offers very high resistance when reverse biased. Because it allows current in one direction only (unidirectional conduction), it converts AC into DC.

    1 mark

Answer: Diffusion of carriers leaves immobile ions forming the depletion layer; their field creates the potential barrier opposing further diffusion. The diode's unidirectional conduction (low forward, high reverse resistance) makes it a rectifier.

Where the marks are

  • Formation of the depletion layer by diffusion leaving immobile ions, with diagram (1 mark)
  • Formation of the potential barrier through the field/drift balance (1 mark)
  • Unidirectional conduction as the feature used in rectification (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 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.