In this paper, we propose a model to describe the transport of trapped particles in a surface potential. It is derived from a microscopic (kinetic) description of particle motion in the potential subject to collisions with the solid surface. Under speciic hypotheses on the collision operator and adequate time and space rescaling, the kinetic model is formally shown to converge to a diiusion equation in position-energy space known in the literature as the SHE model (for Spherical Harmonics Expansion). The model applies to several practical situations in semiconductor and plasma physics. Acknowledgements: The author would express his gratitude to J. P. Boeuf for having suggested this problem and provided encouragements and references.