CBSE Class 12 · 2022 · SemiconductorsShort Answer

(i) Arsenic is pentavalent and donates electrons, so $C_1$ is an n-type semiconductor; aluminium is trivalent and creates holes, so $C_2$ is a p-type semiconductor.
(ii) A pure (intrinsic) semiconductor has very few carriers and poor conductivity.
Doping introduces a large, controlled number of majority carriers, raising the conductivity and making the material useful for diodes and transistors.
$\boxed{C_1:\text{n-type},C_2:\text{p-type};\text{doping raises conductivity.}}$