  Insertion of a resistive contact between a ferromagnetic metal and a
semiconductor microstructure is of critical importance for achieving efficient
spin injection into a semiconductor. However, the equations of the diffusion
theory are rather cumbersome for the junctions including such contacts. A
technique based on deriving a system of self-consistent equations for the
coefficients of spin injection, "gamma", through different contacts are
developed. These equations are concise when written in the proper notations.
Moreover, the resistance of a two-contact junction can be expressed in terms of
"gamma"'s of both contacts. This equation makes calculating the spin valve
effect straightforward, allows to find an explicit expression for the junction
resistance and to prove that its nonequilibrium part is positive. Relation of
these parameters to different phenomena like spin-e.m.f. and the junction
transients is established. Comparative effect of the Coulomb screening on
different parameters is clarified. It is also shown that the spin
non-conservation in a contact can have a dramatic effect on the non-equilibrium
resistance of the junction.
