What is the Correct Definition of Dynamic Resistance and Resistivity?

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Dynamic resistance is defined as Rd = dV/dI, commonly applied to diodes. An alternative expression, Rd = d(IR)/dI, leads to Rd = R + I dR/dI, indicating a relationship with static resistance. The concept of dynamic resistivity lacks a straightforward definition, but it can be expressed as ρd = d(Iρ)/dI, resulting in ρd = ρ + I dρ/dI. Another proposed definition for dynamic resistivity is ρD = dE/dJ, linking it to electric field and current density. Understanding these definitions is crucial for analyzing diode behavior and electrical properties.
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I'm confused about dynamic resistance.

The definition I've seen is Rd = dV/dI.

Is it also ok to write it Rd = d(IR)/dI, so that Rd = R + I dR/dI ?

I couldn't find a straightforward definition for dynamic resistivity at all …

is it ρd = d(Iρ)dI = ρ + I dρ/dI ?
 
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Dynamic resistance is dV / dI. It is often used to refer to the dynamic forward resistance of a diode junction, or the dynamic resistance of a zener diode junction.
 
If RD = dV/dI, then a reasonable definition for dynamic resistivity would be
ρD = dE/dJ,​

where E is electric field and J is the current density.
 
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