  We discuss a three-dimensional (3D) spin drift diffusion (SDD) model to
inject spin from a ferromagnet (FM) to a normal metal (N) or semiconductor
(SC). Using this model we investigate the problem of spin injection into
isotropic materials like GaAs and study the effect of FM contact area and SC
thickness on spin injection. We find that in order to achieve detectable spin
injection a small contact area or thick SC samples are essential for direct
contact spin injection devices. We investigate the use of thin metal films (Cu)
proposed by S.B. Kumar et al. and show that they are an excellent substitute
for tunnelling barriers (TB) in the regime of small contact area. Since most
tunnelling barriers are prone to pinhole defects, we study the effect of
pinholes in AlO tunnelling barriers and show that the reduction in the
spin-injection ratio ($\gamma$) is solely due to the effective area of the
pinholes and there is no correlation between the number of pinholes and the
spin injection ratio.
