Question

Assertion (A): On forward biasing a p-n junction diode, the height of the barrier potential increases. Reason (R): In forward biasing of a p-n junction diode, the direction of the applied voltage is in the same direction as the built-in potential.

• A. Both A and R are true and R explains A
• B. Both A and R are true but R does not explain A
• C. A is true but R is false
• D. A is false and R is false

Hint:

Forward bias reduces the depletion width and barrier potential, thereby increasing current. Reverse bias increases the depletion width and barrier potential, thereby reducing current.

Answer 1

The Correct Option is D

Concept: A p-n junction diode contains a depletion region around the junction. Due to diffusion of charge carriers, a built-in electric field is established across the junction which creates a potential barrier known as the barrier potential. This barrier potential opposes the further diffusion of majority charge carriers. The behaviour of the diode depends upon whether it is forward biased or reverse biased. Assertion Analysis: In forward biasing, \[ p\text{-side} \rightarrow \text{positive terminal} \] and \[ n\text{-side} \rightarrow \text{negative terminal}. \] The external applied voltage opposes the built-in potential barrier. As a consequence, the depletion region becomes thinner and the barrier potential decreases. Because the barrier potential is reduced, majority charge carriers can cross the junction more easily, resulting in a large forward current. Therefore, the statement \[ \text{``barrier potential increases''} \] is incorrect. The barrier potential actually decreases during forward biasing. Hence, the Assertion is FALSE. Reason Analysis: The built-in potential of the junction and the externally applied forward-bias voltage act in opposite directions. The purpose of forward biasing is precisely to oppose and reduce the junction barrier. Therefore the statement that the applied voltage acts in the same direction as the built-in potential is incorrect. In reality, \[ \text{Forward Bias Voltage} \] acts opposite to \[ \text{Built-in Potential}. \] Hence the Reason is also FALSE. Relationship Between Assertion and Reason: Since the Assertion is false and the Reason is also false, the given pair falls under the category: \[ \text{Assertion False, Reason False}. \] Final Answer: \[ \boxed{(D)} \]