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

Given that the Energy band gap of photodiode, E_g = 2.8 eV

Wavelength, λ = 6000 nm = 6000 × 10^–9 m

The energy of a signal is given by the relation:

…. (I)

Where,

h = Planck’s constant = 6.626 × 10^–34J-s

c = Speed of light = 3 × 10⁸m/s

So, E = 3.3 × 10^–20 J [From equation (I)]

We know that, 1.6 × 10^–19J = 1 eV

Therefore,

∴ E = 0.21 eV

The energy of any particular signal of wavelength 6000 nm is 0.21 eV, which is less than given 2.8 eV − the energy band gap of the photodiode. So, the photodiode cannot detect the signal.

(Actually, this is example of application of photodiode)