CBSE Class 12 · 2023 · SemiconductorsShort Answer

(i) A large current produces heating that breaks many covalent bonds, creating a flood of free carriers; the junction then conducts heavily in both directions, loses its rectifying property and may be permanently damaged.
(ii) An intrinsic semiconductor has $n_e=n_h$ and few carriers, giving low conductivity.
Adding a trivalent or pentavalent impurity greatly increases one kind of carrier, raising the conductivity so the material can be used to make devices.
$\boxed{\text{(i) heating breaks bonds; (ii) doping raises carrier density.}}$