The porous nature of a metallic deposition of Pt on a GaAs semiconductor substrate is thought to be responsible for the sensitivity to detection of NH3 gas. In this paper we present a study of the porous character of the Pt film of Pt/GaAs Schottky diodes used as NH3 gas sensors, using a scanning tunnelling microscope (STM). The STM not only allows one to obtain images with high lateral resolution allowing the topographic characterization of the porous size, but also provides the possibility of making electrical measurements in very localized regions of the surface, so giving the possibility of distinguishing between the different materials present on the surface. Both characteristics will allow one to obtain information about the performance of these sensors. With this study we pretend to find a better explanation about the relation between the porous film metallic deposition and the detection sensitivity to NH3 gas by means of the evaluation of the mean grain size. The local electrical behaviour of the discontinuous Pt film allows one to distinguish the porous from the rest of the surface. From this behaviour we have modelled the experimental system and we have seen the importance of the capacitor associated to the depletion layer of the Schottky diodes.