The growing technological challenges and increasing costs are gradually guiding MOS- FET scaling to an end. Hence the fundamental physical limitations will be reached soon, thus preventing further improvements in computational capacity with charged-based devices. The electron spin properties are of immense interest because of their potential for future spin-driven microelectronic devices. Modern charge-based electronics is dominantly fabricated on the material system with silicon, and thus understanding the details of spin propagation in silicon structures and spin manipulation by electrical means is the key for novel spin-based device applications. The spin relaxation in modern silicon field effect transistors is caused by the interplay between the spin-orbit interaction and the electron scattering. Spin relaxation has been noticed to be stronger in thin films but can be substantially suppressed by uniaxial stress. This makes it extremely promising for future applications as the same stress configuration is routinely used to achieve a charge carrier mobility enhancement in modern MOSFETs. To evaluate the spin-flip rates, the wave functions corresponding to the spin-up and spin-down projections with respect to the spin injection direction are needed. The two-band k&p Hamiltonian with the spin-orbit interaction developed near the X-point of the Brillouin zone is used to determine the subband energies and the wave functions in a (001) ultra-thin silicon film under shear strain. The subband wave functions are further used to evaluate the corresponding spin relaxation matrix elements. It is demonstrated that the spin-flip processes between the two  valleys are responsible for spin relaxation in thin (001) silicon films. The enhancement of the spin lifetime is the result of the suppression of intersubband scattering caused by the shear strain induced equivalent  valley splitting. It is further observed that the spin relaxation is sensitive to the spin injection direction, and that the spin lifetime increases to its maximum, when the injection direction is changed from perpendicular-to-plane to in-plane relative to the sample. One of the major criteria to realize the spin-based field-effect transistors (Spin-FETs) is to realize efficient spin injection in silicon. Therefore, the spin injection in silicon from a ferromagnet by electrical means is of great interest. A comprehensive study of the spin drift and spin diffusion in silicon for spin injection from a ferromagnet including electric field effects is performed. In order to avoid the impedance mismatch problem, the spin injecting source is considered to be a ferromagnetic semiconductor. The effect of interface charge screening on spin injection efficiency is under scrutiny. It is noted that the spin current increases while injected from a charge-depleted region in the ferromagnet in comparison to when injected from a charge neutral or an accumulated region. Furthermore it is found that the spin injection efficiency is always limited by the bulk spin polarization of the magnetic material. These results demand a further investigation of the spin transport in silicon, when spin is injected through only a charge neutral and a space-charge layer. The substantial spin transport differences between the spin injection behavior through an accumulation and a depletion layer are investigated, whereas in both cases the spin current density can not be significantly higher than the spin current density at charge neutrality. Therefore, at a fixed bound- ary spin polarization, the maximum spin current is always determined by its value at the charge neutrality condition - provided the charge current is kept constant.