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GATE 2025 EC Q.31: solution and answer

GATE 2025 · ECQ.31 · 1 mark · MSQ

Which of the following can be used as an n-type dopant for silicon? Select the correct option(s).

  1. (A)

    Arsenic

  2. (B)

    Boron

  3. (C)

    Gallium

  4. (D)

    Phosphorous

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Answer and solution

ANSWER

n-type doping of silicon requires pentavalent (Group V) donor atoms; among the options, Arsenic (Group V) and Phosphorous (Group V) qualify, while Boron and Gallium (Group III) are p-type acceptors. Correct options: (A) and (D).

Official answer: A, DSource: Official answer key, GATE 2025 EC (IIT Roorkee)

  1. Silicon is a Group IV (tetravalent) semiconductor: each Si atom shares its 4 valence electrons in covalent bonds with 4 neighboring atoms in the crystal lattice.

  2. n-type doping requires donor impurity atoms with 5 valence electrons (Group V of the periodic table). Four of the five valence electrons satisfy the covalent bonding requirement with neighboring Si atoms; the fifth electron is only loosely bound to its parent atom and easily ionizes at room temperature, donating a free electron to the conduction band.

  3. p-type doping requires acceptor impurity atoms with 3 valence electrons (Group III). Such an atom can form only 3 covalent bonds, leaving one bond incomplete — this incomplete bond acts as a hole that can accept an electron from a neighboring Si-Si bond, creating a mobile hole in the valence band.

  4. Classify each option by its periodic table group: Arsenic (As) — Group V, hence an n-type donor for Si. Boron (B) — Group III, hence a p-type acceptor for Si. Gallium (Ga) — Group III, hence a p-type acceptor for Si. Phosphorous (P) — Group V, hence an n-type donor for Si (the most commonly used n-type dopant in practice).

  5. Therefore, the valid n-type dopants for silicon among the given options are Arsenic and Phosphorous, i.e., options (A) and (D).

KEY IDEA

n-type doping of a Group IV semiconductor (silicon) requires pentavalent (Group V) donor impurities, which contribute a loosely bound extra electron to the conduction band; p-type doping requires trivalent (Group III) acceptor impurities.

WATCH OUT FOR

Confusing donor and acceptor group numbers — assuming Group III elements donate electrons, or forgetting that silicon (Group IV) needs a 5-valence-electron dopant to supply the extra "donor" electron for n-type conduction.

From the GATE 2025 EC paper. Every question of this paper · Download the full paper · All 9 Semiconductors questions, with notes

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