The electronic and chemical properties of G aA s(lll) Surfaces have been investigated at Daresbury Synchrotron Radiation Source in Ultra High Vacuum (UHV) conditions before and after molecular sulphur adsorption. The surface geometry has been determined using Low Energy Electron Diffraction (LEED). The surface composition and electronic structure have been studied using angle integrated and angle resolved photoemission. The objective was to gain an understanding of the chemical and electronic passivation mechanisms resulting from the termination of the G aA s(lll) surfaces by a sulphur monolayer. The results indicate that these systems are complex with evidence of surface and substrate interactions. Passivation work on InP was carried out at Dublin City University. InP samples which had been chemically etched, plasma etched and treated with molecular sulphur have been compared. Simple Schottky barrier devices were fabricated in UHV conditions. The device characteristics were then determined using current-voltage and capacitance-voltage profiles and deep level transient spectroscopy. There are indications that this surface treatment may well prove to have practical applications in device fabrication.