  We consider spin polarized transport in a
ferromagnet-insulator/semiconductor/insulator-ferromagnet (F1-I-S-I-F2)
junction. We find that the spin current is strongly dependent on the spin
configurations, the doping and space charge distribution in the semiconductor.
When the ferromagnet-semiconductor interface resistance is comparable to the
semiconductor resistance, the magnetoresistance ratio of this junction can be
greatly enhanced under appropriate doping when the space charge effect in the
nonequilibrium transport processes is taken into consideration.
