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Foundation5 past questions

Diode types and applications

Explore the working, characteristics, and uses of different diodes such as Zener diodes, including their role in voltage regulation and breakdown phenomena.

Why this shows up in the exam

NEET frequently asks about the operation and practical uses of diodes in circuits.

How NEET tests this

Statement analysis · 3 QsDirect recall · 3 QsNumerical

Learn the idea

A Zener (or avalanche) breakdown occurs when a reverse‑biased p‑n junction is driven to its breakdown voltage; the junction then clamps the voltage at that value while the reverse current rises sharply. The key insight is that in breakdown the voltage is essentially fixed, not the current.

🧠 Memory hook: Zener = Zero change in voltage, big surge in current – think of a ‘Z‑clamp’ that holds voltage steady while current floods in

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

Formulas & facts to keep ready

  • Breakdown voltage (V_BR) is the reverse voltage at which a sudden increase in reverse current occurs
  • Zener breakdown (V_BR < 5 V) is caused by quantum‑mechanical tunnelling of electrons
  • Avalanche breakdown (V_BR > 5 V) is caused by carrier multiplication through impact ionisation
  • A Zener diode is operated in reverse bias near its breakdown voltage to give a stable reference voltage
  • In the breakdown region the I‑V curve is nearly vertical – a small change in voltage produces a large change in current
  • Voltage regulation is obtained by connecting a series resistor with the Zener diode

How to approach it

  1. 1Read the question carefully and note the bias condition (forward or reverse) and the voltage value mentioned
  2. 2Recall which breakdown mechanism is relevant (Zener for low V_BR, avalanche for high V_BR) and that the voltage across the diode stays essentially constant at V_BR
  3. 3Use the characteristic that current increases sharply while voltage is clamped to eliminate wrong options
  4. 4Select the option that matches “voltage constant, current rises” or apply the same reasoning to any other diode‑type question

Worked example — watch it click

In a reverse-biased p-n junction, when the applied bias voltage is equal to the breakdown voltage, then:

  • A)current remains constant while voltage increases sharply.
  • ✅voltage remains constant while current increases sharply.
  • C)current and voltage increase.
  • D)current and voltage decrease.

The concept behind this problem

The worked example asks exactly what happens at the breakdown point of a reverse‑biased p‑n junction, testing the student’s grasp of the voltage‑clamp, current‑rise behaviour that defines Zener (or avalanche) breakdown

Step by step

  1. 1At breakdown voltage in reverse bias, avalanche or Zener breakdown occurs.
  2. 2The voltage across the junction remains essentially constant (clamped at breakdown voltage) while current increases sharply, limited only by external circuit resistance.
  3. 3This is the principle of Zener diodes used as voltage regulators.

Watch out

Students often mistakenly pick the option that says the current remains constant while the voltage rises, forgetting that breakdown fixes the voltage, not the current.

Common slip-ups that cost marks

  • •Confusing the forward‑bias conduction region with reverse‑bias breakdown
  • •Assuming the reverse current stays constant at breakdown instead of the voltage
  • •Mixing up Zener and avalanche breakdown voltage ranges

🌟 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 5

In a reverse-biased p-n junction, when the applied bias voltage is equal to the breakdown voltage, then:

Push further

More challenging

5 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 5

Consider a Zener diode operating in its breakdown region. Which of the following statements accurately describes its voltage-current relationship under these conditions?

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