  Extraction of electrons from a semiconductor to a ferromagnet as well as the
case of injection in the reverse direction may be formulated as a scattering
theory. However, the presence of bound states at the interface arising out of
doping on the semiconductor side must be taken into account in the scattering
theory. Inclusion of the interface states yields an explanation of a recent
result of spin imaging measurement which contradicts the current understanding
of spin extraction. The importance of an extraction theory to spintronics is
illustrated by an application to a spin switch.
