Achieving electrical control of spin injection in semiconductors is highly desirable for spin-based quantum information processing. By inserting a tunnel barrier between a ferromagnet and a semiconductor, electrical injection and detection of spin have been demonstrated in various systems using the Hanle technique. However, overshadowed by the possible effect of impurity states embedded in the tunnel barrier, origin of the measured signal remains a topic of hot debate. In this talk, we present without a priori a direct comparison of the magneto-resistive response between spin injection into conduction band and that into the impurity states, in which the difference is remarkable. First, I will show that growth of crystalline BaTiO3 directly atop p-type Ge using advanced molecular beam epitaxy results in an atomically abrupt oxide-semiconductor interface, offering a viable means of mitigating the effect of impurity states when BaTiO3 is used as the tunnel barrier. By operating the device in the quantum tunneling region, we extract a spin lifetime ~ 102 ps, longer by two orders of magnitude than previous results. Furthermore, I will demonstrate control of the spin signal by manipulating the band profile across the device, which modulates the charge transport between quantum tunneling and impurity-assisted conduction, effectively acting as a “switch” for spin injection. This approach points to a new paradigm of spin-dependent phenomena in semiconductor-based tunnel devices via band profile tuning.