  In spin-polarized bipolar transport both electrons and holes in doped
semiconductors contribute to spin-charge coupling. The current conversion
between the minority (as referred to carriers and not spin) and majority
carriers leads to novel spintronic schemes if nonequilibrium spin is present.
Most striking phenomena occur in inhomogeneously doped magnetic {\it p-n}
junctions, where the presence of nonequilibrium spin at the depletion layer
leads to the spin-voltaic effect: electric current flows without external bias,
powered only by spin. The spin-voltaic effect manifests itself in giant
magnetoresistance of magnetic {\it p-n} junctions, where the relative change of
the magnitude of electric current upon reversing magnetic field can be more
than 1000%. The paper reviews nonmagnetic and magnetic spin-polarized {\it p-n}
junctions, formulates the essentials of spin-polarized bipolar transport as
carrier recombination and spin relaxation limited drift and diffusion, and
discusses specific device schemes of spin-polarized solar cells and magnetic
diodes.
