Question $4(20$ marks$)($a$)$ Explain how the drift current in semiconductors is generated. $[4$ marks$]($b$)$ Explain which drift current component is dominant in: $($i$)$ n$-$type semiconductors. $[3$ marks$]($ii$)$ p$-$type semiconductors. $[3$ marks$]($c$)$ Given that at temperature of $300$K$,$ an electric field applied to an extrinsic gallium arsenide sample causes equal electron and hole current densities. Determine: $($i$)$ the ratio of the equilibrium hole density to the equilibrium electron density. $[3$ marks$]($ii$)$ the equilibrium electron and hole densities. $[4$ marks$]($iii$)$ the net doping concentration. $[3$ marks$]$