We demonstrate electrical spin injection in GaAs from ferromagnetic metals through tunneling. We use and compare an AlOx insulating layer as well as the native Schottky barrier at the metal-semiconductor interface as tunnel barriers. The injected spin polarization has been measured by analyzing the electroluminescence polarization using the oblique Hanle effect technique. The observed bias dependence of the measured spin polarization is sensitive to both the type of injector and the doping profile in the semiconductor. Hot electron spin relaxation due to the D'yakonov-Perel spin relaxation mechanism has been identified as the cause of the different bias dependence in two AlOx-based injectors with different doping levels of the active region while a change in the electron transit times in the highly doped interfacial region has been found as the cause of the bias dependence in the Schottky-based injectors. (C) 2004 American Vacuum Society.