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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 current would experience more or less voltage drop (equal to the barrier height at that time) 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?

tryingtobeastoic
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    @AJN, What is vague in "the current would experience more or less voltage drop"? OP has said it very well. There is an exciting input voltage source VIN that passes a current (via a resistor R) through the forward-biased PN junction. The opposing voltage VF across the junction is subtracted from VIN and, in this way, the effective current-creating voltage VIN - VF decreases... so the current I = (VIN - VF)/R decreases as well. – Circuit fantasist Jun 13 '21 at 10:12
  • I see that, makes sense. I will delete the comment. – AJN Jun 13 '21 at 12:52
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    @V.V.T, There is a misconception here. *"A resistance equal to the slope of the v/i curve at the operating point"* is "differential", not "dynamic" resistance. "Dynamic" is something else - the ratio VA/IA at the operating point A what is called "static (ohmic) resistance". It is static but when either the voltage or current changes, it also changes... so it is dynamic. It sounds strange but "static" is what is dynamic here. Note that the differential resistance may not change when voltage/current varies (if the IV curve is linear in this region). OP correctly senses it with their intuition... – Circuit fantasist Jun 13 '21 at 12:56
  • @Circuitfantasist So, resistance (in general) is the differentiation of V with respect to I at the operating point of a VI graph. For non-ohmic materials, this resistance is dynamic and thus the graph isn't linear. – tryingtobeastoic Jun 13 '21 at 13:38
  • @user545735, It is said that Om originally formulated his law in such a differential form... and after finding that for a copper conductor the relation is constant, he presented it in our familiar integral form. – Circuit fantasist Jun 13 '21 at 14:05

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Let's assume that the diode is direct or reverse biased by a DC voltage source and some static current is flowing.

The dynamic resistance of a diode is the resistance experienced by a small signal time varying voltage source that is in series to the DC bias source.

The small signal voltage source is a sine wave at a very low frequency. The frequency must be low because we want to measure R and not C.

Z = R + jωC

We want ω very low in order to have the term ωC negligible with respect to R.

It's called small signal resistance.

As you say, it is related to the widening and narrowing of the depletion region.

Enrico Migliore
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