In a photodiode, the conductivity increases when the material is exposed to light. It is found that the conductivity changes only if the wavelength is less than 620 nm. What is the band gap?
The conduction if take place if the energy provided to the electron is more equal or more than the band gap.
Given:
Maximum wavelength at which the conduction starts 
∴ this wavelength provides minimum energy to the electron to move to conduction band,
Energy of this radiation=Energy band gap
∴ energy band gap = 2.003eV ≅ 2eV
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