  We study the effect of the impurity density on lifetimes and relaxation
lengths of electron spins in the presence of a static electric field in a
n-type GaAs bulk. The transport of electrons and the spin dynamics are
simulated by using a semiclassical Monte Carlo approach, which takes into
account the intravalley scattering mechanisms of warm electrons in the
semiconductor material. Spin relaxation is considered through the
D'yakonov-Perel mechanism, which is the dominant mechanism in III-V
semiconductors. The evolution of spin polarization is analyzed by computing the
lifetimes and depolarization lengths as a function of the doping density in the
range 10^{13} - 10^{16} cm^{-3}, for different values of the amplitude of the
static electric field (0.1 - 1.0 kV/cm). We find an increase of the electron
spin lifetime as a function of the doping density, more evident for lattice
temperatures lower than 150 K. Moreover, at very low intensities of the driving
field, the spin depolarization length shows a nonmonotonic behaviour with the
density. At the room temperature, the spin lifetimes and depolarization lengths
are nearly independent on the doping density. The underlying physics is
analyzed.
