Explain, with the help of a circuit diagram, the working of a photo-diode. Write briefly how it is used to detect the optical signals. 

OR

Mention the important considerations required while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of band gap of an LED if it is required to emit light in the visible range?              

In any diode, an electric field ‘E’ exists across the junction from n-side to p-side, when light with energy h greater than energy gap E_g illuminates the junction. Then electron-hole pairs are generated due to absorption of photons, in or near the depletion region of the diode. Due to existing electric field, electrons and holes get separated. The free electrons are collected on n-side and holes are collected on p-side, giving rise to an emf.

Due to this generated emf, an electric current of A order flows through the external resistance.

The figure above is the circuit diagram of photodiode.

Detection of optical signals:

Photodiode can be used to detect optical signals if we observe the change in current with change in light intensity when a reverse bias is applied.
                                                         OR

Important considerations in the fabrication of LED:

i) LED is heavily doped p-n junction.

ii) Reverse breakdown voltage of LED’s are very low, typically around 5 V.

The order of band gap of an LED to emit light in the visible range is about 3 eV to 1.8 eV.