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2.9

Semiconductors: P-type and N-type

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

Semiconductors: P-type and N-type

Phones, laptops and cars all use semiconductors.

Their secret is the electron configuration of silicon.

Smart Syllabus 2026 leaves out this section (2.9) for the 2026 exam. Read it for understanding, not for marks.

1 · Semiconductors and silicon

Semiconductors conduct electricity under some conditions.

The book names silicon, germanium and arsenic as examples.

Silicon is 2, 8, 4. It has four valence electrons.

In a pure crystal each Si atom bonds to four other Si atoms. Then no electrons are free to conduct.

2 · P-type and N-type

P-type: add atoms with three valence electrons, like aluminium.

Silicon cannot make four bonds there. Holes form. Holes act as positive charge carriers.

N-type: add atoms with five valence electrons, like phosphorus.

Silicon makes four bonds. The fifth electron is extra. It is free to move and carries negative charge.

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Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

Semiconductors: P-type and N-type

Phones, laptops and cars all use semiconductors.

Their secret is the electron configuration of silicon.

Smart Syllabus 2026 leaves out this section (2.9) for the 2026 exam. Read it for understanding, not for marks.

1 · Semiconductors and silicon

Semiconductors conduct electricity under some conditions.

The book names silicon, germanium and arsenic as examples.

Silicon is 2, 8, 4. It has four valence electrons.

In a pure crystal each Si atom bonds to four other Si atoms. Then no electrons are free to conduct.

2 · P-type and N-type

P-type: add atoms with three valence electrons, like aluminium.

Silicon cannot make four bonds there. Holes form. Holes act as positive charge carriers.

N-type: add atoms with five valence electrons, like phosphorus.

Silicon makes four bonds. The fifth electron is extra. It is free to move and carries negative charge.

Key terms

Semiconductor
A material that conducts electricity under some conditions.
P-type
Made by adding atoms with 3 valence electrons. Holes carry the charge.
N-type
Made by adding atoms with 5 valence electrons. Extra electrons carry the charge.

Short questions with model answers

  1. Q1. Which impurity makes silicon N-type: aluminium or phosphorus?

      Phosphorus. It has 5 valence electrons. Four make bonds. The fifth is free.

    Common mistakes

    • ✗ “Doping with aluminium adds free electrons.”

      ✓ Aluminium has only 3 valence electrons. It makes holes, not extra electrons.

    MCQs

    1. 1. An impurity with five valence electrons gives:

      1. (a) N-type
      2. (b) P-type
      3. (c) a metal
      4. (d) an insulator
      Show answer

      (a) Extra electrons make N-type.

    2. 2. What carries charge in P-type silicon?

      1. (a) holes
      2. (b) neutrons
      3. (c) nuclei
      4. (d) ions only
      Show answer

      (a) Holes act as positive carriers.

    3. 3. Pure silicon: each atom bonds to:

      1. (a) two atoms
      2. (b) three atoms
      3. (c) four atoms
      4. (d) six atoms
      Show answer

      (c) Four other Si atoms.

    Quick revision

    • Silicon has four valence electrons.
    • Three-electron impurity: holes, P-type. Five-electron impurity: free electrons, N-type.

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