  The effect of the interface roughness on the spin injection from a
ferromagnet into a semiconductor is studied theoretically. Even a small
interface irregularity can lead to a significant enhancement of the injection
efficiency. When a typical size of the irregularity, a, is within a domain
lambda_F << a << lambda_N, where lambda_F and lambda_N are the spin-diffusion
lengths in the ferromagnet and semiconductor, respectively, the geometrical
enhancement factor is ~lambda_N. The origin of the enhancement is the
modification of the local electric field on small scales ~a near the interface.
We demonstrate the effect of enhancement by considering a number of
analytically solvable examples of injection through curved
ferromagnet-semiconductor interfaces. For a generic curved interface the
enhancement factor is ~lambda_N / R, where R is the local radius of curvature.
