CBSE Class 12 Physics: Marking Scheme Solution
(a) Majority and Minority Charge Carriers [1 Mark]
- p-type semiconductor: Majority carriers are holes ($h^+$), and minority carriers are electrons ($e^-$).
- n-type semiconductor: Majority carriers are electrons ($e^-$), and minority carriers are holes ($h^+$).
(b) Formation of Diffusion and Drift Currents [2 Marks]
Step 1 (Definition of Diffusion Current): [0.5 Mark]
Diffusion current arises due to the concentration gradient of charge carriers across the p-n junction. Holes diffuse from the p-side to the n-side, and electrons diffuse from the n-side to the p-side. This movement constitutes the diffusion current ($I_{diff}$), which flows from the p-side to the n-side.
Step 2 (Definition of Drift Current): [0.5 Mark]
Drift current arises due to the electric field ($E$) established in the depletion region. This field sweeps the minority carriers (electrons from p-side and holes from n-side) across the junction. This movement constitutes the drift current ($I_{drift}$), which flows from the n-side to the p-side.
Step 3 (Equilibrium Condition): [1 Mark]
In an unbiased p-n junction, the diffusion current and drift current are equal in magnitude but opposite in direction, resulting in zero net current:
$$I_{net} = I_{diff} - I_{drift} = 0$$Thus, the junction reaches a state of dynamic equilibrium where the net current is zero.
💡 EL10 Concept Insight: Think of diffusion like a crowded room where people naturally spread out to empty spaces, while drift is like a strong wind (the electric field) blowing a few stray people back in the opposite direction; eventually, the "spreading out" and the "wind" balance each other perfectly.