Concept:At thermal equilibrium without an external circuit, dynamic balance is established where majority carrier diffusion current is equal and opposite to minority carrier drift current.
Formula:$$J_{\text{total}} = J_{\text{diffusion}} + J_{\text{drift}} = 0 \implies J_{\text{diffusion}} = -J_{\text{drift}}$$
Solution:- Majority carriers diffuse across the junction due to the concentration gradient, creating \( I_{\text{diffusion}} \).
- This diffusion exposes fixed ions that create a built-in electric field, which sweeps thermally generated minority carriers across the junction, creating \( I_{\text{drift}} \).
- At equilibrium, these two currents are equal in magnitude and opposite in direction, resulting in a net current of zero.
Why other options are incorrect:- Option B: Thermal energy maintains continuous random carrier motion at room temperature.
- Option C: The potential barrier reaches a non-zero equilibrium value (\( V_0 \approx 0.7\text{ V} \) in Si).
- Option D: Thermal generation continuously creates new electron-hole pairs throughout the bulk material.
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