The $V$-$I$ characteristic of a silicon diode is shown in the figure. Calculate the resistance of the diode at
(a) $I_D=15mA$ and
(b) $V_D=-10V$.

We know that,

Resistance of diode is given vy $\frac{△V}{△I}$

So, at $I_V=15mA$

From $V-I$characteristic equation:

$R=\frac{\left(0.8-0.7\right)V}{\left(20-10\right)mA}=10\Omega$

And,at $V_n=-10V$

$△I=0$

So, $R$ is infinite.