Question: In forward and reverse biased silicon p-n junction, the dominant mechanisms for the motion of charge carriers are:

Step 1 (Governing Principles of p-n Junction): [0.5 Mark]

• Forward Bias: The external potential $V$ opposes the built-in potential $V_{bi}$. This reduces the depletion barrier, allowing majority charge carriers (electrons from n-side and holes from p-side) to cross the junction. This flow is driven by the concentration gradient, known as diffusion.
• Reverse Bias: The external potential $V$ supports the built-in potential $V_{bi}$. This increases the depletion barrier, blocking majority carriers. However, the electric field $E$ across the junction sweeps minority charge carriers across, a process known as drift.

Step 2 (Analysis of Mechanisms): [0.25 Mark]

Comparing the physical processes:

• Forward bias current $I_f \approx I_{diffusion}$
• Reverse bias current $I_r \approx I_{drift}$

Step 3 (Final Concluded Answer): [0.25 Mark]

Based on the analysis, the correct option is (A) diffusion in forward bias, drift in reverse bias.

💡 EL10 Concept Insight: Think of the p-n junction like a gate: in forward bias, you push the gate open so the crowd (majority carriers) can walk through naturally (diffusion), but in reverse bias, the gate is locked tight, and only a few stragglers (minority carriers) get pulled through by a strong magnetic-like force (drift).