  We analyze the proper inclusion of electric-field-induced spin splittings in
the framework of the envelope function approximation. We argue that the Rashba
effect should be included in the form of a macroscopic potential as diagonal
terms in a multiband approach rather than the commonly used Rashba term
dependent on k and electric field. It is pointed out that the expectation value
of the electric field in a subband is sometimes not unique because the
expectation values can even have opposite signs for the spin-split subband
components. Symmetric quantum wells with Dresselhaus terms and the influence of
the interfaces on the spin splitting are also discussed. We apply a well
established multiband approach to wide modulation-doped InGaSb quantum wells
with strong built-in electric fields in the interface regions. We demonstrate
an efficient mechanism for switching on and off the Rashba splitting with an
electric field being an order of magnitude smaller than the local built-in
field that determines the Rashba splitting. The implications of our findings
for spintronic devices, in particular the Datta-Das spin transistor and
proposed modifications of it, are discussed.
