Concept:In forward bias, external energy lowers the potential barrier, allowing majority carriers (holes from the p-side and electrons from the n-side) to diffuse across the junction, recombining as they enter the opposite regions and establishing a large forward current.
Formula:$$I = I_s \left( e^{\frac{qV}{\eta k_B T}} - 1 \right)$$
Solution:- Forward bias lowers the built-in potential barrier from \(V_0\) to \((V_0 - V)\).
- Majority holes in the p-region and majority electrons in the n-region obtain sufficient thermal energy to overcome the reduced barrier and diffuse across the junction.
- Thus, forward current is primarily a majority carrier diffusion current.
Why other options are incorrect:- Option A: Minority carriers moving under the junction field cause reverse saturation leakage current.
- Option C: Direct bandgap tunneling is significant only in heavily doped Zener/tunnel diodes, not standard forward-biased diodes.
- Option D: Thermionic emission is the operating principle of vacuum tubes, not solid-state \(\text{p-n}\) junctions.
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