(i) A heavy doping makes the depletion region very thin. This makes the electric field of the junction very high, even tor a small reverse bias voltage. This in turn helps the Zener diode to act as a 'voltage regulator'.
(ii) When operated under reverse bias, the photodiode can detect the changes in current with the changes in light intensity more easily.
(iii) The photon energy, of visible light photons varies
from about 1.8 eV to 3 eV. Hence for visible LED's, the semiconductor must have a band gap of 1.8 eV.