When p-type and n-type semiconductor regions are joined, majority carriers initially diffuse across the boundary: electrons move from the n side toward the p side and holes move in the opposite direction.

Near the junction they recombine, leaving behind fixed ionized dopant atoms. The resulting region contains few mobile carriers and is called the depletion region.

The uncovered positive and negative ions create an internal electric field. That field opposes further diffusion and establishes a built-in potential across the junction.

An applied voltage changes this balance. Forward bias lowers the barrier and allows a large carrier flow across the junction. Reverse bias raises the barrier and leaves only a small leakage current until breakdown mechanisms become important.

The p-n junction is the physical structure behind ordinary semiconductor diodes and also appears inside many transistors and optoelectronic devices.