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A p-type Si semiconductor is… (Class 12 Physics 2025, Q21)

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

CBSE Class 12 Physics 2025: question 21

Set in the CBSE Class 12 board exam of 2025, paper 55/1/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerSemiconductor Electronics: Materials, Devices and Simple Circuits

A p-type Si semiconductor is made by doping an average of one dopant atom per silicon atoms. If the number density of silicon atoms in the specimen is atoms , find the number of holes created per cubic centimetre in the specimen due to doping. Also give one example of such dopants.

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

One acceptor atom contributes one hole. How many dopant atoms are there per m³?

Hint 2

Divide the Si number density by , then remember that 1 m³ = 10⁶ cm³.

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. Each trivalent dopant atom creates one hole. Holes per m³ . Since , holes per cm³ .

    1 mark

  2. Example of such a (trivalent/acceptor) dopant: aluminium, boron, gallium or indium.

    1 mark

Answer: 10¹⁵ holes per cm³; dopant e.g. aluminium (or B, Ga, In)

Where the marks are

  • Calculates m⁻³ and converts it to holes per cm³ (1 mark)
  • Names a trivalent dopant such as Al, B, Ga or In (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 55/1/1: 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.