Explain the working of photo diode …

Photodiode: It is a pn junction whose function is controlled by the light allowed to fall on it. Suppose, the wavelength is such that the energy of a photon, hc/λ, is sufficient to break a valance bond. When such light falls on the junction, new hole-electron pairs are created. The number of charge carriers increases and hence the conductivity of the junction increases. If the junction is connected in some circuit, the current in the circuit is controlled by the intensity of the incident light.
Zener diode: It is a diode meant to operate in the breakdown region. In a zener diode the breakdown occur by direct breaking of valance bonds due to high electric field. Large number of electron hole pairs created due to the breakdown takes part in conduction. Once the breakdown occurs, the potential difference across the diode doesn’t increase even if the applied battery potential is increased. Such diodes are used to obtain constant voltage output.