  We present a theoretical method to calculate the expectation value of spin in
an inversion-asymmetric two-dimensional (2D) system with respect to an
arbitrarily spin-polarized electron state, injected via an ideal point contact.
The 2D system is confined in a [001]-grown quantum well, where both the Rashba
and the Dresselhaus spin-orbit couplings are taken into account. The obtained
analytical results allow more concrete description of the spatial behaviors of
the spin precession caused by individually the Rashba and the Dresselhaus
terms. Applying the calculation on the Datta-Das spin-FET, whose original
design considers only the Rashba effect inside the channel, we investigate the
possible influence due to the Dresselhaus spin-orbit coupling. Concluded
solution is the choice of +-[1+-10], in particular [110], as the channel
direction.
