  In semiconductor spintronic devices, the semiconductor is usually lightly
doped and nondegenerate, and moderate electric fields can dominate the carrier
motion. We recently derived a drift-diffusion equation for spin polarization in
the semiconductors by consistently taking into account electric-field effects
and nondegenerate electron statistics and identified a high-field diffusive
regime which has no analogue in metals. Here spin injection from a ferromagnet
(FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying
this spin drift-diffusion equation to several typical injection structures such
as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field
regime spin injection from a ferromagnet into a semiconductor is enhanced by
several orders of magnitude. For injection structures with interfacial
barriers, the electric field further enhances spin injection considerably. In
FM/NS/FM structures high electric fields destroy the symmetry between the two
magnets at low fields, where both magnets are equally important for spin
injection, and spin injection becomes locally determined by the magnet from
which carriers flow into the semiconductor. The field-induced spin injection
enhancement should also be insensitive to the presence of a highly doped
nonmagnetic semiconductor (NS$^+$) at the FM interface, thus FM/NS$^+$/NS
structures should also manifest efficient spin injection at high fields.
Furthermore, high fields substantially reduce the magnetoresistance observable
in a recent experiment on spin injection from magnetic semiconductors.
