In a PN junction, the depletion region is an insulating region within a conductive material where the flow of charge carriers is decreased over time, resulting in a region that is either empty of charge carriers or full of immobile charge carriers. The depletion region forms instantaneously across a PN junction when it is in thermal equilibrium or in a steady state. When a PN junction is formed, some of the free electrons in the N-region diffuse across the junction and combine with holes in the P-region to form negative ions, leaving behind positive ions at the donor impurity sites. Due to the majority charge carrier diffusion, the depletion region is charged, with the N-side positively charged and the P-side negatively charged. The depletion region acts as a barrier that opposes the flow of electrons from the N-side to the P-side of the semiconductor diode. The depletion region is also known as the depletion layer, depletion zone, junction region, or space charge region.