Ambipolar drift of spatially separated electrons and holes

Abstract

We report on the ambipolar transport process of photogenerated, spatially separated charge carriers in the doping layers of a $p\ensuremath{-}i\ensuremath{-}n$ diode under the influence of lateral electric fields. By including externally applied electric fields into the theory of ambipolar diffusion of spatially separated electrons and holes, we show that the transport of excess carriers can be described as a combined drift and diffusion process. Compared to the well-known ambipolar transport in bulk material, the ambipolar diffusion process is enhanced by several orders of magnitude, whereas the ambipolar drift can be described by the same ambipolar mobility as in bulk material if the electric fields in both doping layers are identical. One major difference of the ambipolar drift of electrons and holes propagating in different layers in comparison to bulk material is the possibility to control the ambipolar mobility dynamically by changing the dark carrier densities by varying the reverse bias applied to the $p\ensuremath{-}i\ensuremath{-}n$ structure. Most interesting however, is the fact that the ambipolar drift can be controlled by different external fields for electrons and holes. In order to verify the predictions of our theoretical description of the ambipolar transport of spatially separated electrons and holes, we have developed a new pump-and-probe technique that allows for a direct temporally and spatially resolved investigation of the various transport scenarios. The results agree very well with the theoretical simulations.