The dynamic resistance of a diode is defined as:
**Depletion region** also called space-charge region, thickness typically 0.5 μm, contains no mobile carriers, only fixed ions, negative on p-side, positive on n-side, diffusion current due to concentration gradient, drift current due to field, equilibrium when J_drift + J_diff =0, voltage drop mainly across depletion region. Dynamic resistance ( r_d ) is the ratio of a small change in voltage ( Δ V ) to the corresponding small change in current ( Δ I ), i.e., r_d = Δ V / Δ I , reflecting the diode's behavior under small signal variations. Substituting values gives Δ V / Δ I, which matches exp
Ref: NCERT > Physics Book > Electronic Devices > p-n Junction, Depletion Region and Diode Characteristics
