In this paper we have analyzed the possibility of enhancing spin-polarization performance of conventional nonmagnetic semiconductor heterostructures which rely on the resonant tunneling mechanism. Both the bulk inversion asymmetry (BIA) and the structural inversion asymmetry (SIA) effects are taken into account in the presented model. The aim is to engineer nanostructures with maximal degree of spin separation in the electron tunneling current, which might be useful in studying various spin-related phenomena in semiconductor materials. Spin-polarization status of the current, in the devices under consideration, should be controllable by moderate emitter-collector voltages. Additionally, the spin orbit-interactions affect the dwell times of electrons in spin-up and spin-down states, therefore the prospects of spin-filtering in the time domain may be considered as well.