$4.$

$($a$)$ A p$-$n junction diode has a diffusion capacitance of $20$ pF at a forward bias of $0\mathrm{.}7$ V $,$ with a minority carrier lifetime of $\backslash (0\mathrm{.}5\backslash$mu $\backslash$mathrm$\{$~s$\}\backslash ).$ If the bias is changed to $0\mathrm{.}6$ V $,$ please calculate the new diffusion capacitance value, and estimate the change in stored charge. $(8$ points$)$