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Foundation12 past questions

p-n junction formation and characteristics

Study how p-n junctions are formed, the creation of the depletion region, barrier potential, and how these properties change with biasing and temperature.

Why this shows up in the exam

Questions often focus on the structure, behavior, and response of p-n junctions under different conditions.

How NEET tests this

Statement analysis · 6 QsDirect recall · 4 QsNumerical · 3 Qs

Learn the idea

A p‑n junction creates a built‑in (barrier) potential because electrons and holes diffuse and leave behind fixed ions, forming a depletion region. The magnitude of this potential is set by the semiconductor material, the doping concentrations, and the temperature.

🧠 Memory hook: V₀ is a three‑ingredient soup: temperature (kT), doping (N_A N_D), and the secret spice n_i (material) – all must be in the recipe.

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

Formulas & facts to keep ready

  • Barrier potential V₀ = (kT/e) ln(N_A N_D / n_i²) (NCERT Eq.)
  • Depletion region is free of mobile carriers and contains ionised donors (n side) and acceptors (p side)
  • Under forward bias the external voltage reduces V₀; under reverse bias it adds to V₀
  • Intrinsic carrier concentration n_i depends on band‑gap → depends on the semiconductor material
  • Increasing temperature raises n_i exponentially, thereby lowering V₀
  • Higher doping (larger N_A or N_D) raises V₀ because the logarithm term increases

How to approach it

  1. 1Write the standard expression for the built‑in potential
  2. 2Identify every variable that appears in that expression
  3. 3Match each variable to the physical factor asked (material → n_i, doping → N_A,N_D, temperature → T)
  4. 4Select the option that includes all present factors

Worked example — watch it click

The barrier potential of a p-n junction depends on : (1) type of semiconductor material (2) amount of doping (3) temperature Which one of the following is correct?

  • A)(2) and (3) only
  • ✅(1), (2) and (3)
  • C)(1) and (2) only
  • D)(2) only

The concept behind this problem

The worked example checks whether you recall the V₀ formula and recognise that each term – temperature, doping, and intrinsic carrier concentration (material) – directly influences the barrier potential.

Step by step

  1. 1Barrier potential V₀ = (kT/e)ln(N_A N_D/n_i²).
  2. 2It depends on: (1) semiconductor material (through n_i and band gap), (2) doping levels N_A and N_D, and (3) temperature T.
  3. 3All three factors affect barrier potential.

Watch out

Students often omit the material factor, forgetting that n_i (and thus the band‑gap) makes the barrier potential material‑dependent.

Common slip-ups that cost marks

  • •Confusing the applied bias voltage with the built‑in potential
  • •Assuming temperature only changes V₀ linearly and ignoring its effect on n_i
  • •Dropping the material dependence by overlooking n_i’s role

🌟 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 12NEET 2015

In the given figure, a diode D is connected to an external resistance R =100 Ω and an e.m.f of 3.5V. If the barrier potential developed across the diode is 0.5 V, the current in the circuit will be: D 100Ω R 3.5 V

Push further

More challenging

26 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 26

A circuit consists of a 10 V battery, a silicon p-n junction diode, and a 1.5 kΩ resistor connected in series. Assuming the diode is ideal for reverse bias and has a constant forward voltage drop of 0.7 V, what is the current flowing through the circuit if the battery polarity is reversed?

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