Question

Give reasons for the following :

(i) The Zener diode is fabricated by heavily doping both the p and n sides of the junction.

(ii) A photo diode, when used as a detector of optical signals is operated under reverse bias.

(iii) The band gap of the semiconductor used for

fabrication of visible LED'* must at least be 1.8 eV

Answer 1

(i) A heavy doping makes the depletion region very thin. This makes the electric field of the junction very high, even for a small reverse bias voltage. This in turn helps the Zener diode to act as a 'voltage regulator

(ii) When operated under reverse bias, the photo diode can detect the changes in current with the changes in light intensity more easily.

(iii) The photon energy, of visible light photons varies from about 1.8 cV. Hence for visible LED'*, the semiconductor must have a band gap of 1.8 eV.