Question

When light falls on the junction of a diode, the number of charge carriers ____ and hence the conductivity of the junction ____.
A) Increases, decreases
B) Decreases, increases
C) Decreases, decreases
D) Increases, increases

Answer 1

We know that PN Junction diode is a basic semiconductor that controls flow of electrons and hence, electric current flows in a circuit. Now, light falls on the junction diode, the light gets absorbed by this junction. Hence, more numbers of electron-hole pairs are generated. Falling light rays decrease the resistance of the PN junction diode.

Complete answer:
A PN junction diode is a two-electrode semiconductor device, which allows the flow of electric current in the same direction. It blocks the flow of electric current in the opposite direction or reverse direction and if the diode is forward biased then it allows the electric current to flow.

We also know that the electron-hole pair is the function unit of generation in inorganic semiconductors. An electron hole also known as hole is the lack of an electron at a position where one electron could exist. Now, in semiconductors, the free charge carriers that are electron-hole pairs are created by excitation of electrons from the valence band to the conduction band.

When light rays above a certain frequency having certain threshold energy are incident on the junction diode, the width of the depletion layer increases. The electrons in the depletion layer absorb the photons of the light and excite from the bond and thus they create an electron-hole pair. Therefore as the number of charge carriers increases and hence the conductivity also increases.

Therefore, (D) is the correct option.

Note: As we know, a PN junction diode can act in both forward bias as well as reverse bias.
It can allow electric current to flow as well as block the current to flow in the reverse direction.
PN junction is used as a solar cell, LED lightning application, rectifiers and many other electrical components.