Dynamic resistance: A P-N junction diode allows electric current in one direction and blocks electric current in another direction. It allows electric current when it is forward biased and blocks electric current when it is reverse biased.
When forward biased voltage is applied to the P-N junction diode, the width of depletion region decreases. However the depletion region cannot be completely vanished. There exists a thin depletion region or depletion layer in the forward biased diode. Therefore, a thin depletion region and atoms in the diode offer some resistance to electric current. This resistance is called forward resistance.
In a forward biased p-n junction diode two types of resistances takes place based on the voltage applied.
(i) Static resistance or DC resistance
(ii) Dynamic resistance or AC resistance