By incorporating spin-dependent properties and magnetism in semiconductor structures, new applications can be considered which go beyond magnetoresistive effects in metallic systems. Notwithstanding the prospects for spin/magnetism-enhanced logic in semiconductors, many important theoretical, experimental, and materials challenges remain. Here we discuss the challenges for realizing a particular class of associated applications and our proposal for bipolar spintronic devices in which carriers of both polarities (electrons and holes) would 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.
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