Unipolar transient carrier transport in a semiconductor heterostructure with two subbands

The effective diffusivity in a multi-component ensemble is not only determined by the partial diffusion coefficients of the constituents, but also by different diagonal and off-diagonal elements of the velocity matrix. Different velocities of the respective subbands give rise to a positive excess contribution to the effective diffusion coefficient. However, a reduction of diffusivity can arise, when the off-diagonal elements of the velocity tensor differ from each other. In this case, the cross correlation between subbands favors the tendency towards phase separation that manifests itself in a qualitative change of the transient carrier transport. Under this condition, memory effects have a profound influence on the transport properties.