Diffusion current refers to the flow of charge carriers (such as electrons or holes in a semiconductor) due to a concentration gradient. In a material, charge carriers tend to move from regions of high concentration to regions of low concentration, similar to how substances diffuse from areas of higher concentration to areas of lower concentration in a fluid.
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A diffusion current results from a spatial carrier density gradient. For semiconductor electrons and holes in one dimension, conventional-current densities are and . Their different signs reflect negative and positive carrier charge. At equilibrium each species' diffusion current is balanced by its drift current, giving zero net current while the individual transport contributions need not vanish.