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GATE ECE — free MCQ practice

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In an intrinsic semiconductor at room temperature, the concentration of electrons equals:Read the answer and worked solution →In an n-type semiconductor, majority carriers are:Read the answer and worked solution →Mass-action law in semiconductors:Read the answer and worked solution →Energy gap of silicon at 300 K is approximately:Read the answer and worked solution →

Worked practice questions

In an intrinsic semiconductor at room temperature, the concentration of electrons equals:

  1. Number of donors
  2. Number of atoms
  3. Zero
  4. Concentration of holes (n = p = n_i)
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Correct answer: D. Concentration of holes (n = p = n_i)

**Intrinsic semiconductor:** pure, no dopants. Thermal excitation creates electron-hole pairs in equal numbers. n = p = n_i (intrinsic carrier concentration). **For Si at 300K:** n_i ≈ 1.5 × 10¹⁰ /cm³.

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In an n-type semiconductor, majority carriers are:

  1. Both equal
  2. Electrons (donor atoms like P, As donate free electrons)
  3. Ions
  4. Holes
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Correct answer: B. Electrons (donor atoms like P, As donate free electrons)

n-type: doped with pentavalent (P, As, Sb). Donor atoms contribute extra electrons. **Majority:** electrons (n >> p). Minority: holes. Conductivity dominated by electron motion.

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Mass-action law in semiconductors:

  1. n = p
  2. n + p = constant
  3. n × p = n_i² (regardless of doping)
  4. n - p = 0
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Correct answer: C. n × p = n_i² (regardless of doping)

**Law of mass action:** n × p = n_i² (constant at given temperature). **For n-type:** n ≈ N_d, p = n_i²/N_d (small). **For p-type:** p ≈ N_a, n = n_i²/N_a. Independent of doping level.

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