The V-I characteristics of a practical diode is diffrent from its ideal diode.In practical the V-I characteristics depends on its internal static and dynamic resistances.
In above figure V-I characteristics of semiconductor diodes Si and Ge are shown.
From the above figure we can observe as the forward current is negligible until the voltage across the diode reaches to Vcutin .
Cut-in volatge: It is the minimum voltage required for the diode to start conducting.The cut-in voltage for Silicon (Si) diode is 0.7V and for Germanium (Ge) it is 0.3V.
In the reverse characteristics of diode , the current is almost constant at room temperature.
Static Resistance of a diode in Forward Bias:
Statics resistance is defined at the specified operating point of diode.
Rd = V/I
Dynamic Resistance of a diode in Forward Bias:
Dynamic resistance of a diode is defined as the slope of the characteristics at the operating point.
rd = ΔV/ΔI