Question

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?

Answer 1

i) pn junction diode should be connected in forward biased condition.

ii) The energy of photon visible light is given by $hv=E_g$, never eg is the bend gap between balance q conduction band and $v$ is the field freq. of emitted visible radiation wavelength of emitted light $\frac{hv}{\lambda }=Eg$ or

$\lambda =\frac{hc}{Eg}$

iii)Led should be made of compounds like $GaAs$ with $Eg$ a $1.9ev$ to emit light in visible region.

2)To emit light in visible range in Led, band gap is of order$1.8ev$ (minimum).

because spectral range of visible light is $0.4\times {10}^{-6}m$ to $0.7\times {10}^{-6}m$ with energy $3ev$ to $1.8ev$.