Photodiode- Photodiode is a special type of junction diode used for detecting optical signals. It is a reverse biased-n junction made from a photosensitive material. Its symbol is.

A photodiode fabricated with a transparent cover to allows light to fall on the diode and operated under reverse bias. A photodiode is used in sensor circuits.

(i) The current in the forward bias is known to be more (~mA) than the current in the reverse bias(~mA). What is the reason to operate the photodiode in reverse bias? 

(ii) Three photodiodes ${\mathrm{D}}_1,{\mathrm{D}}_2 \mathrm{and} {\mathrm{D}}_3$ are made of semiconductors having band gaps of 1.5 eV,2.5 eV and 35. eV, respectively. Which of them will not be able to detect light of wavelength450 nm? 

(iii) Write main use of photodiode.

(i)                  When photodiode is illuminated with light due to breaking of covalent bonds, equal number of additional electrons and holes come into existence whereas fractional change in minority charge carrier is much higher than fractional change in majority charge carrier.

Since, the fractional change of minority carrier current is measurable significantly in reverse bias than that of forward bias. Therefore, photodiodes are connected in reverse bias.

(ii)         $$\begin{gathered} \mathrm{Given}, \mathrm{\lambda }=450 \mathrm{nm} \\ \mathrm{Energy}, \mathrm{E}=\frac{1240}{\mathrm{\lambda }}\mathrm{\AA }=\frac{1240}{450}\mathrm{eV}=2.66 \mathrm{eV} \end{gathered}$$So, diodes $D_2$ and $D_3$ will not detect light wave of wavelength 450 nm.       

 (iii)               Main use of photodiode The photodiode are extensively used in high speed reading of computer, punched cards, light-detection systems, light-operated switches, counting of objects interrupting a light beam, etc.