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Exam level6 past questions

Semiconductor basics and doping

Learn about intrinsic and extrinsic semiconductors, doping processes, charge carrier types, and how doping affects electrical properties.

Why this shows up in the exam

NEET tests your understanding of how semiconductors are made and how their properties are controlled for electronic applications.

How NEET tests this

Assertion–reason · 3 QsDirect recall · 3 QsApplication

Learn the idea

Intrinsic semiconductor has equal electrons and holes; doping replaces some lattice atoms with impurities that either donate extra electrons (n‑type) or create holes (p‑type). The key insight is to look at the impurity’s valence compared to the host crystal.

🧠 Memory hook: P‑donor gives Plenty of electrons; A‑acceptor creates Holes – P for n‑type, A for p‑type

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Intrinsic semiconductor: pure crystal, equal electron‑hole concentration
  • Extrinsic semiconductor: doped crystal, majority carriers determined by impurity
  • Pentavalent impurity (group V) → donor → n‑type
  • Trivalent impurity (group III) → acceptor → p‑type
  • Free electrons are majority carriers in n‑type; holes are majority in p‑type
  • Conductivity increases because carrier concentration rises

How to approach it

  1. 1Read the question and note which element is used as dopant
  2. 2Compare its group (valence electrons) with that of the semiconductor (Ge or Si are group IV)
  3. 3If dopant has 5 valence electrons → donor → n‑type; if 3 → acceptor → p‑type
  4. 4Use this to pick the correct statement or option

Worked example — watch it click

An intrinsic semiconductor is converted into n-type extrinsic semiconductor by doping it with:

  • ✅Phosphorous
  • B)Aluminium
  • C)Silver
  • D)Germanium

The concept behind this problem

The worked example asks which impurity makes an n‑type semiconductor; recognizing that a pentavalent donor supplies an extra free electron directly tests the donor‑impurity rule.

Step by step

  1. 1An n-type semiconductor is created by doping an intrinsic semiconductor with pentavalent impurity atoms (donors).
  2. 2Pentavalent atoms have 5 valence electrons; when they replace a semiconductor atom in the crystal lattice, 4 electrons form covalent bonds and the 5th electron becomes a free electron, increasing electron concentration.
  3. 3Phosphorous (P) is a pentavalent element (Group 15).
  4. 4Aluminium is trivalent (creates p-type), Silver is monovalent, and Germanium is tetravalent (intrinsic semiconductor itself).

Watch out

Choosing Aluminium because it is a metal, forgetting that it is trivalent and therefore creates p‑type, not n‑type.

Common slip-ups that cost marks

  • •Confusing group number with metallic nature – a metal like Al is trivalent, not a “metallic” donor
  • •Mixing up majority and minority carriers in p‑type vs n‑type
  • •Assuming any added impurity changes conductivity without checking its valence

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 6NEET 2011

If a small amount of antimony is added to germanium crystal:

Push further

More challenging

12 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 12

Assertion A: For semiconductor devices, materials with larger band gaps are generally preferred for high-power and high-frequency applications. Reason R: A larger band gap implies lower intrinsic carrier concentration and higher breakdown voltage.

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