Surface models for perpendicular ambipolar transport in kinetic and hydrodynamic theories

The relevant boundary conditions in kinetic and hydrodynamic theories are investigated for stationary ambipolar transport perpendicular to the surface. In a first approach, the surface recombination velocity is calculated from microscopic reflection probabilities assuming bulk distribution functions for the carriers moving towards the surface. However, because the distribution function near the surface may differ significantly from its bulk shape, the profiles of the hydrodynamic variables from a kinetic calculation do not agree, in the kinetic boundary layer, with those from a hydrodynamic calculation: This boundary layer is calculated by numerical solution of the Boltzmann equation. The hydrodynamic profiles are found to depend not only on the total reflection probability but also on the detailed probability density in velocity space. For very small reflection probabilities the model of a heated displaced Maxwellian distribution has no solution up to the boundary because of the existence of an upper bound for the drift velocity.