As the diffusion continues, the depletion region begins to form and the energy level of the n-region conduction band decreases.
The decrease in the energy level of the conduction band in the n region is due to the loss of the higher-energy electrons that have diffused across the junction to the p region.
Soon, there are no electrons left in the n-region conduction band with enough energy to get across the junction to the p-region conduction band, as indicated by the alignment of the top of the n-region conduction band and the bottom of the p-region conduction band in Figure 1–20(b).
At this point, the junction is at equilibrium; and the depletion region is complete because diffusion has ceased.
There is an energy gradient across the depletion region which acts as an “energy hill” that an n-region electron must climb to get to the p region.