In order to create $$semi$-$insulating" GaAs, a $0\mathrm{.}5$ mm thick wafer $($Eg $=1$:$42$ eV$)$ is doped with NCr $=4*1018/$cm$3$ of Cr$,$ which has an acceptor level $0\mathrm{.}67$eV above the valence band. The Fermi level is located $0\mathrm{.}6$eV above the valence band. $($a$)$ How many Cr atoms are ionized? $(5$pt$)($b$)$ How many active carriers can be found in Conduction band?