The increase in the width of the depletion region in a p-n junction diode is due to :

Concept:

Depletion region: 

• The region near the p-n junction where the flow of charge carriers (free electrons and holes) is reduced over a given period and finally results in zero charge carriers.
• The width of the depletion region depends on the number of impurities added to the semiconductor.

• When a p-n junction is forward-biased:
  • The charge carriers are pushed towards the junction.
  • If the voltage of the battery is large enough diode-electrons reach the p-end and fill the holes.
  • So, The depletion layer is eliminated, and a current passes through the diode.
  • While the battery continues to supply electrons for the n-end, this removes electrons from the p-end, which works as supplying holes.
  • With a further increase in voltage from the battery, the current increases.

• When a p-n junction is reverse biased:
  • Both the free electrons in the n-type semiconductor and the holes in the p-type semiconductor are attracted toward the battery.
  • The width of the depletion layer is increased, and no charge carriers meet.
  • Almost no current passes through the diode.
  • It acts like a very high ohm resistor or the same as an insulator. 

EXPLANATION:

• From the above explanation, we can see that, when a diode is forward-biased, then the depletion layer width decreases, and the barrier height is reduced which will leads to the conduction of electrons through the junction.
• Similarly, when a diode is reverse biased, then the depletion layer width increases. Therefore option 4 is correct.