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 Kumar et al. [Appl. Phys. Lett. 91, 142110 (2007)] and show that they are an excellent substitute for tunneling barriers (TB) in the regime of small contact area. Since most tunneling barriers are prone to pinhole defects, we study the effect of pinholes in AlO tunneling barriers and show that the reduction in the spin-injection ratio (γ) 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.