  An impressive success of spintronic applications has been typically realized
in metal-based structures which utilize magnetoresistive effects for
substantial improvements in the performance of computer hard drives and
magnetic random access memories. Correspondingly, the theoretical understanding
of spin-polarized transport is usually limited to a metallic regime in a linear
response, which, while providing a good description for data storage and
magnetic memory devices, is not sufficient for signal processing and digital
logic. In contrast, much less is known about possible applications of
semiconductor-based spintronics and spin-polarized transport in related
structures which could utilize strong intrinsic nonlinearities in
current-voltage characteristics to implement spin-based logic. Here we discuss
the challenges for realizing a particular class of structures in semiconductor
spintronics: our proposal for bipolar spintronic devices in which carriers of
both polarities (electrons and holes) contribute to spin-charge coupling. We
formulate the theoretical framework for bipolar spin-polarized transport, and
describe several novel effects in two- and three-terminal structures which
arise from the interplay between nonequilibrium spin and equilibrium
magnetization.
