A comparative study of diode structures based on semi-insulating (SI) GaAs with evaporated Mg vs. Pt contacts is presented. Electric field strength in the vicinity of the contacts is inferred from the detection of alpha-particles emitted from Am-241 radioisotope. It is shown that at zero bias, the structure with Mg-based contacts (MgO/SI GaAs) gives comparatively small signal implying an ohmic behaviour. Contrary, the structure with Pt contacts reveals presence of a non-negligible electric field near the contacts at zero bias voltage, thus indicating presence of a sizeable space charge region near the interface. The results are supplemented by insights from atomistic electronic structure modeling of idealized GaAs/Pt vs. GaAs/Mg and GaAs/MgO interfaces.