When a p-n junction is reverse-biased, the applied voltage adds to the built-in potential barrier. The external electric field pulls the majority charge carriers (holes in the p-region and electrons in the n-region) away from the junction. This causes the width of the depletion region to increase. In contrast, during forward bias, the applied voltage opposes the built-in potential barrier, which pushes the majority carriers towards the junction and decreases the width of the depletion region.