The cause of the potential barrier in a PN diode is ______?

In a P-N junction diode, when the p-type and n-type semiconductors are brought together, the majority charge carriers (holes in the p-type and electrons in the n-type) diffuse across the junction due to their concentration gradients. This diffusion causes the region near the junction to become depleted of mobile charge carriers, leaving behind fixed positive ions in the p-type and negative ions in the n-type. These immobile ions create an electric field that opposes further diffusion of charge carriers, forming a potential barrier. This depletion region is what causes the potential barrier in the PN junction.