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We theoretically study the spin-dependent transport of conduction electrons across typical metal/semiconductor (SC)/metal structures, where the SC channel exhibits Rashba spin-orbit coupling (SOC) and the metal contacts do not. The spatial discontinuity of the Rashba SOC is shown to result in highly localized, effective magnetic field barriers at the device interfaces. As a result, electrons with oppositely polarized spins along the injection direction are found to be transmitted with different probabilities, resulting in a finite spin polarization. The value of the spin polarization depends sensitively on the Rashba SOC strength within the SC channel, which is well known to be adjustable by an applied gate bias. Thus the proposed structure could be useful as a tunable source of spin-polarized current in spintronic applications.