The p-n junction is the most basic building block of semiconductor electronics.  It consists of a p-type material in perfect contact with an n-type material.  The area within the vicinity of the junction is known as the depletion region, because it is depleted of mobile carriers (electrons and holes).  This is because the electrons from the n-type material have crossed the junction and diffused into the other side (p-type), recombining with holes in that side.  On the other hand, the holes from the p-type material have diffused to the n-type material, recombining with electrons.

Because of this diffusion process, holes not covered by electrons are left in the n-type material, while electrons not covered by holes are left in the p-type material. Known as uncovered charges, these result in an over-all negative charge in the p-type material and an over-all positive charge in the n-type material. 

This separation of charges develops a potential across the depletion region, preventing further diffusion of carriers across the junction.  This potential, known as the potential barrier, is about 0.6- 0.7V in a typical silicon p-n junction. A voltage greater than this potential barrier has to be applied across the p-n junction in order to make current flow through the junction.  This characteristic of the p-n junction is the basis for the operation of a device known as the junction diode.