Explain working of photodiode and zener 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.
photodiode is a pn junction diode operated in reversee bias. it has a transparent window at the depletion region. when light of suitable energy greater than the energy gap of the semiconductor is incident on the junction, then electron hole pairs are created due to absorption of photons. due to electric field of the junction, electrons and holes are separated before they can recombine. the direction of the electric field is such that electrons reach the n-side and holes to the p-side. this gives rise to an emf and subsequently a photocurrent.
zener diode is a special purpose pn junction diode designed to work in the reverse breakdown region and is used as a voltage regulator.. it is heaily doped on both n and p sides of the junction. therfore depletion region is very thin and electric field of the junction is very high even for a small reverse bias voltage. when the reverse bias voltage becomes greater than breakdown voltage there is large change in current even for insignificant change in voltage. this property is used in voltage regulators.