Barrier Potential
Any time there is a positive charge and a negative charge near each other, there is a force acting on the charges as described by Coulomb’s law.
In the depletion region there are many positive charges and many negative charges on opposite sides of the pn junction.
The forces between the opposite charges form an electric field, as illustrated in Figure 1–19(b) by the blue arrows between the positive charges and the negative charges.
This electric field is a barrier to the free electrons in the n region, and energy must be expended to move an electron through the electric field. That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region.