Class 12 Physics Semiconductor Electronics Photodiode


Application – Photodiodes are used for detecting optical signal or they act as photo detectors

Circuit Diagram:

  • This is a special p-n junction diode made up of photosensitive semiconducting material
  • The diode has a transparent window to allow light to fall on the diode


  • It is operated under reverse bias below the breakdown voltage
  • When photodiode is illuminated with light (photons) , with energy hט greater than the energy gap of the semiconductor, the electron-hole pairs are generated due to the absorption of photons, in or near the depletion region of the diode
  • Due to electric field at the junction, electrons and holes are separated before they combine
  • The direction of electric field is such that the electrons reach n-side and the holes reach the p-side
  • The movement of electrons and holes gives rise to emf
  • When external load is connected, current flows. The magnitude of the photocurrent depends on the intensity of incident light
  • In reverse bias, we can observe that as intensity increases, the current also increases


The characteristic of a photodiode is shown below:

Problem : A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 6000 nm?

Solution : Eg = 2.8 eV; λ = 600 nm = 600 * 10-9

We know, Energy E = hc / λ

 = 6.6 * 10-34 * 3 * 108 / 600 * 10-9 * 1.6 * 10-19 eV

 = 2.06 eV

As E < Eg, 2.06 < 2.8 eV so p-n junction cannot detect the radiation of given wavelength

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