Q12
1 markMCQSection A

In an unbiased p-n junction, at equilibrium, which of the following statements is true?

Semiconductor Electronics: Materials, Devices and Simple Circuits
p-n Junction at Equilibrium — Drift and Diffusion

Options

(A)Diffusion current is zero but drift current exists.
(B)Diffusion current exists but drift current is zero.
(C)Diffusion and drift currents are equal and opposite.
(D)Both the diffusion and drift currents exist but are unequal.
Official Answer

Correct option: (C) Diffusion and drift currents are equal and opposite.


In an unbiased junction at equilibrium:


  • Diffusion current flows (p → n) as majority carriers cross due to concentration gradients.
  • Drift current flows (n → p) as minority carriers are swept by the barrier field.
  • The barrier potential adjusts until the two are equal in magnitude and opposite in direction, so the net current is zero.
p-n junction equilibriumdiffusion currentdrift currentequal and oppositezero net currentdepletion regionbarrier potentialunbiased junction

Marking Scheme

  • 11 mark: correct option (C) diffusion and drift currents equal and opposite.
  • 2Key reasoning: net current must be zero without a bias, so the two currents balance rather than disappear.

Hint

No battery means zero net current — so the two internal currents must cancel, not vanish.

Quick Oral Answer

In an unbiased p-n junction the diffusion current of majority carriers and the drift current of minority carriers are equal and opposite, so the net current across the junction is zero.

Analysis & Explanation

This checks understanding of dynamic equilibrium at an unbiased p-n junction.


Concept: Two currents coexist across the depletion region:

  • Diffusion current — majority carriers (holes from p, electrons from n) diffuse across because of the large concentration gradient; directed from p-side to n-side.
  • Drift current — minority carriers are pushed by the built-in electric field of the depletion layer; directed from n-side to p-side.

At equilibrium the growing barrier potential slows diffusion until the diffusion and drift currents exactly cancel:

  • both currents exist and flow continuously, but they are equal in magnitude and opposite in direction, giving zero net current with no external supply.

Why (C) is correct: it states this balance precisely.


Why the others are wrong:

  • (A) and (B) wrongly claim one current vanishes — both are present at equilibrium.
  • (D) says the currents are unequal; if they were unequal there would be a net current with no battery, which is impossible in a device at equilibrium.

Exam trap: Equilibrium does not mean the currents stop; it means they balance.

Common Mistakes

  1. 1Believing one of the currents becomes zero at equilibrium — both diffusion and drift currents are actually present.
  2. 2Thinking equilibrium means no charge motion; it means the two opposite currents cancel to give zero net current.
  3. 3Selecting (D) unequal currents, which would imply a net current flowing without any external source.

Interesting Facts

The built-in potential that balances these currents is about 0.3 V for germanium and 0.7 V for silicon at room temperature.

This same drift–diffusion balance, when disturbed by light, is what generates the photovoltage in a solar cell.

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Frequently Asked Questions

Does the net current in an unbiased p-n junction really become zero?

Yes. Without an external bias there can be no net current. The diffusion current (majority carriers) and drift current (minority carriers) still flow but are equal in magnitude and opposite in direction, so they cancel exactly.

What maintains the balance between diffusion and drift currents?

The built-in barrier potential of the depletion region. As majority carriers diffuse across, the growing barrier field increases the opposing drift current until the two currents are equal and equilibrium is reached.