This chapter introduces the concept of incoherent spin current. A diffusive spin current can be driven by spatial inhomogeneous spin density. Such spin flow is formulated using the spin diffusion equation with spin-dependent electrochemical potential. The chapter also proposes a solution to the problem known as the conductivity mismatch problem of spin injection into a semiconductor. A way to overcome the problem is by using a ferromagnetic semiconductor as a spin source; another is to insert a spin-dependent interface resistance at a metal–semiconductor interface.
