Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Diffusion current in a p-n junction is greater than the drift current in magnitude if the junction is forward biased.
Reason R: Diffusion current in a p-n junction is from the n-side to the p-side if the junction is forward biased.

At any p-n junction two carrier-transport mechanisms exist simultaneously:

• Diffusion current, produced by the gradient in majority-carrier concentration.
• Drift current, produced by the electric field inside the depletion region.

Under thermal equilibrium the two currents are equal and opposite, so the net current is zero:

$$J_{\text{diff}} + J_{\text{drift}} = 0$$ $$-(1)$$

Forward bias applied

• The external forward voltage $$V_F$$ lowers the junction (barrier) potential $$V_0 - V_F$$.
• A lower barrier allows many more majority carriers (electrons from the n-side and holes from the p-side) to cross the junction by diffusion.
• The diffusion current therefore increases exponentially with the applied forward voltage: $$J_{\text{diff}} \propto e^{\frac{qV_F}{kT}}$$.

• The drift current depends mainly on the magnitude of the electric field inside the depletion region. That field decreases only slightly when the diode is forward biased, so $$J_{\text{drift}}$$ changes very little.

Consequently, in forward bias

$$\left|J_{\text{diff}}\right| \gt\!\!\left|J_{\text{drift}}\right|$$ $$-(2)$$

Statement A is therefore correct.

Direction of the diffusion current in forward bias

• Electron concentration is high on the n-side and low on the p-side. Electrons diffuse from the n-side towards the p-side.
• Hole concentration is high on the p-side and low on the n-side. Holes diffuse from the p-side towards the n-side.
• For conventional current we add the two contributions. Electrons moving n→p represent conventional current in the same n→p direction (negative charge moving n→p is equivalent to positive current n→p with a minus sign, but the two carrier types together give a net diffusion current that we describe as flowing from the n-side to the p-side).

Hence, when the junction is forward biased, the diffusion current is considered to flow from the n-side to the p-side. This makes Assertion R correct.

Relation between R and A

The very fact that majority carriers diffuse from the side of higher concentration (n) to the side of lower concentration (p) explains why their diffusion component grows rapidly once the barrier is lowered, and therefore why it overtakes the drift component in magnitude. Thus R provides the physical reason for A.

Both Assertion A and Reason R are correct, and R is the correct explanation of A.
Hence, the correct option is Option D.