How to solve

$7\mathrm{.}8$ An $n^{+}$diffusion is used for interconnection. The surface concentration of the diffusion is $4\frac{{10}^{19}}{c}{m}^3$ and the junction depth is $4m.$ The diffusion is formed in a $p-$type wafer with a background concentration of $1\frac{{10}^{15}}{c}{m}^3.$

$($a$)$ What is the sheet resistance of this diffusion?

$($b$)$ Estimate the capacitance per unit length if the diffusion is $15m$ wide. Assume the rectangular geometry shown in Fig. P$7\mathrm{.}8$ and use the step$-$junction$-$capacitance formula.

FIGURE P$7\mathrm{.}8$