# Is the dynamic resistance of a diode due to the narrowing and widening of the depletion zone?

All diodes have a built-in voltage. When a diode is connected under forward or reverse bias, the depletion region either widens or narrows. Obviously, the barrier height also increases or decreases respectively. So, if a current was flowing through the diode under forward bias and the voltage across the circuit was fluctuating due to a small AC voltage in addition to the DC biasing voltage, the voltage drop of the circuit there (equal to the barrier height at that time) would be more or less with time. Thus, we can say that the resistance of the diode due to the barrier height is increasing or decreasing with time due to the fluctuation of the small AC signal in addition to the DC biasing voltage. Is this called the dynamic resistance of a diode?

• @Matt, could you look into this question? Jun 11, 2021 at 14:22

It is an interesting phenomenon that the dynamic resistance is the resistance that the AC voltage sees, since dynamic resistance, $$r=\frac{\Delta V}{\Delta I}$$ and the AC voltage is the voltage that is changing. To speak more clearly, $$\Delta V$$ is the voltage of the AC source at that time and $$\Delta I$$ is the current due to that voltage.