Problem $7\mathrm{.}31(10$ points$)$ The total junction capacitance of a GaAs pn junction at T $=300$ K is found to be $1\mathrm{.}10$ pF at VR $=1$ V$.$ The doping concentration in one region is measured and found to be $8$ x $1016$ cm$3,$ and the built$-$in potential is found to be Vbi $=1\mathrm{.}20$ V$.$ Determine $($a$)$ the doping in the other region of the pn junction and $($b$)$ the cross$-$sectional area. $($c$)$ The reverse$-$bias voltage is changed and the capacitance is found to be $0\mathrm{.}80$ pF$.$ What is the value of VR$?$