  Novel spin transport behavior is theoretically shown to result from replacing
the usual metal (or poly-silicon) gate in a silicon field-effect transistor
with a ferromagnet, separated from the semiconductor by an ultra-thin oxide.
The spin-dependent interplay between the drift current (due to a source-drain
bias) and the diffusion current (due to carrier leakage into the ferromagnetic
gate) results in a rich variety of spin dependence in the current that flows
through such a device. We examine two cases of particular interest: (1)
creating a 100% spin-polarized electrical current and (2) creating a pure spin
current without a net electrical current. A spin-valve consisting of two
sequential ferromagnetic gates is shown to exhibit magnetoresistance dependent
upon the relative orientations of the magnetization of the two ferromagnets.
The magnetoresistance ratio grows to arbitrarily large values in the regime of
low source-drain bias, and is limited only by the spin-flip time in the
channel.
