Concept:Once the applied forward bias voltage exceeds the junction threshold (knee voltage), the potential barrier is largely overcome, allowing large increases in forward current for very small increases in voltage, resulting in a very low forward dynamic resistance.
Formula:$$r_f = \frac{\Delta V_f}{\Delta I_f}$$
Solution:- Beyond the threshold voltage (\(0.7\text{ V}\) for Si, \(0.3\text{ V}\) for Ge), the \(I\)-\(V\) characteristic curve becomes nearly vertical.
- The dynamic resistance \(r_f = \Delta V_f / \Delta I_f\) falls to a range of approximately \(1\text{ to }25\,\Omega\).
Why other options are incorrect:- Option A: Mega-ohm resistance is characteristic of a reverse-biased diode.
- Option B: Kilo-ohm resistance occurs only at voltages well below the knee threshold.
- Option C: A real diode contains bulk semiconductor and contact resistance, so resistance is never identically zero.
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