We investigate conductance properties in ballistic spin field-effect transistors (SFETs) by taking into account the Rashba spin-orbit coupling (SOC), interface scattering, the presence of an in-plane magnetic field, band mismatch, and spin polarization in the ferromagnetic electrodes. It is shown that the conductance of the SFET has high peaks for high potential barriers at the contact/channel interfaces. Therefore, switching on or off can be easily realized by tuning either the band mismatch, the Rashba SOC strength, or the magnitude or direction of the magnetic field. Moreover, in the SFET with Ohmic-contact interfaces, the conductance modulation results from a mixing between Fabry-Perot-type spin channels interference and spin precession and becomes more and more pronounced as the spin polarization in the contacts increases.
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