Investigations on light emission by inelastic tunneling in metal-insulator-semiconductor structures, as concept for a new type of sensor, were performed. Tunneling junctions were designed, fabricated and characterized in terms of elec-trical performance and as source of electromagnetic radiation. The light emission process is a two stages process. Fluc-tuations of the current generate propagating surface plasmons, which are subsequently scattered by the structure rough-ness at the boundaries.