Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R

Assertion A: A diode under reverse-biased condition provides very small current which is nearly independent of voltage until a critical limit at which the current increases drastically.

Reason R: Below the critical voltage limit, only majority charge carriers flow which increases drastically above critical voltage.
Choose the correct answer from the options given below : 

In a $$p$$-$$n$$ junction diode the biasing conditions decide the direction and magnitude of current.
Let the applied voltage be $$V$$ with the convention that forward bias is positive and reverse bias is negative.

Behaviour under reverse bias
  • When the junction is reverse-biased, the depletion region widens and the majority carriers are repelled away from the junction.
  • The only carriers that are able to cross the junction are the thermally generated minority carriers (electrons in the $$p$$-region and holes in the $$n$$-region).
  • Their concentration is fixed by temperature, not by the magnitude of the reverse voltage (as long as the electric field is below the breakdown limit).
  • Hence the reverse current remains almost constant and is called the reverse saturation current $$I_S$$.

When the reverse voltage magnitude becomes large enough, the electric field in the depletion region exceeds a critical value. Avalanche multiplication (or Zener tunnelling, depending on doping) starts, causing a sudden, very large increase in reverse current. This point is called the breakdown voltage $$V_{BR}$$.

Verification of Assertion (A)
The statement in (A) says: “A reverse-biased diode gives a small current independent of voltage until a critical limit is reached.”
This is exactly the description of the constant reverse saturation current up to the breakdown voltage. Therefore Assertion (A) is true.

Verification of Reason (R)
Reason (R) states: “Below the critical voltage, only majority carriers flow.”
This is incorrect because, under reverse bias, majority carriers are blocked by the widened potential barrier. The small reverse current is due to minority carriers, not majority carriers. Hence the Reason (R) is false.

Relationship between A and R
Since (A) is true and (R) is false, (R) cannot be the correct explanation of (A).

Therefore, the correct choice is:
Option C which is: A is true but R is false.