Ambipolar diffusion is diffusion of positive and negative species with opposite electrical charge due to their interaction via an electric field. In the case of ionic crystals, the fluxes of the diffusing species are coupled, while in a plasma the various species diffuse at the same rate.
的所经过的距离称作双极性扩散长度（Ambipolar diffusion length）。理论上，它可以用公式表达为
，其中
为双极性扩散系数,
为电子（空穴）的寿命。