Correct Option: (C) Diffusion and drift currents are equal and opposite.
Step 1 (Governing Principle & Definition): [1/3 Mark]
In an unbiased p-n junction at equilibrium, the net current ($I_{net}$) flowing through the junction must be zero. The two primary mechanisms contributing to current flow are:
- Diffusion Current ($I_{diff}$): Caused by the concentration gradient of charge carriers (holes from p-side to n-side, electrons from n-side to p-side).
- Drift Current ($I_{drift}$): Caused by the electric field ($E$) in the depletion region, which sweeps minority carriers across the junction.
Step 2 (Mathematical Working): [1/3 Mark]
Since the junction is unbiased and in thermal equilibrium, there is no external potential difference applied. Therefore, the net current across the junction is given by the algebraic sum of the two components:
$$I_{net} = I_{diff} + I_{drift} = 0$$Rearranging the equation to satisfy the condition of equilibrium:
$$I_{diff} = -I_{drift}$$This indicates that the magnitude of the diffusion current is exactly equal to the magnitude of the drift current, but they flow in opposite directions.
Step 3 (Final Concluded Answer): [1/3 Mark]
Because the currents are equal in magnitude and opposite in direction, they cancel each other out, resulting in a net current of zero. Thus, statement (C) is the correct physical description of an unbiased p-n junction.
💡 EL10 Concept Insight: Imagine a room with a door where people are naturally wandering from a crowded side to an empty side (diffusion), but a security guard is pushing a few people back the other way (drift). At equilibrium, the number of people wandering out exactly matches the number of people being pushed back in, so the total flow through the door stays at zero!