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Recently, an electron-spin filter was proposed by depositing a nanosized ferromagnetic metal stripe and a Schottky normal metal stripe on the top of the semiconductor heterostructure [F. Zhai, H. Q. Xu and Y. Guo, Phys. Rev. B70 (2004) 085308]. This device has a considerable electron-spin polarization and potential application in spintronics. Here we apply a bias to this device and theoretically demonstrate how to manipulate its electron-spin polarization. By numerical simulations, we found that not only the amplitude of the electron-spin polarization but also its sign varies with the bias, giving rise to a bias-tunable spin filter device. [ABSTRACT FROM AUTHOR]