  A theory of spin-polarized electron transport in ferromagnet-semiconductor
heterostructures, based on a unified semiclassical description of ballistic and
diffusive transport in semiconductors, is outlined. The aim is to provide a
framework for studying the interplay of spin relaxation and transport mechanism
in spintronic devices. Transport inside the (nondegenerate) semiconductor is
described in terms of a thermoballistic current, in which electrons move
ballistically in the electric field arising from internal and external
electrostatic potentials, and are thermalized at randomly distributed
equilibration points. Spin relaxation is allowed to take place during the
ballistic motion. For arbitrary potential profile and arbitrary values of the
momentum and spin relaxation lengths, an integral equation for a spin transport
function determining the spin polarization in the semiconductor is derived. For
field-driven transport in a homogeneous semiconductor, the integral equation
can be converted into a second-order differential equation that generalizes the
spin drift-diffusion equation. The spin-polarization in ferromagnet
semiconductor structures is obtained by matching the spin-resolved chemical
potentials at the interfaces, with allowance for spin-selective interface
resistances. Illustrative examples are considered.
