$3.[20$ Points$]$ Find the free electron concentration, hole concentration, and the conductivity of theGaAs crystal of a$,$ b and c as follows.a$.$ Pure GaAs crystal at $27$ OC b$.$ GaAs crystal doped with $2$x$1015$ cm$-3$ donors at $27$ OC c$.$ GaAs crystal doped with $2$x$1015$ cm$-3$ donors and $9$x$1014$ cm$-3$ acceptors at $27$ OC d$.$ Find the lowest Fermi energy level of the crystal in a b and c$.($answer in eV$.)(5$ points$)(5$ points$)(5$ points$)(5$ points$)$