  We provide a theoretical framework for the electric field control of the
electron spin in systems with diffusive electron motion. The approach is valid
in the experimentally important case where both intrinsic and extrinsic
spin-orbit interaction in a two-dimensional electron gas are present
simultaneously. Surprisingly, even when the extrinsic mechanism is the dominant
driving force for spin Hall currents, the amplitude of the spin Hall
conductivity may be considerably tuned by varying the intrinsic spin-orbit
coupling via a gate voltage. Furthermore we provide an explanation of the
experimentally observed out-of-plane spin polarization in a (110) GaAs quantum
well.
