Controlling the threshold voltage of a metal–oxide–semiconductor field effect transistor by molecular protonation of the Si:SiO2 interface

Abstract
We present results from a hybrid molecular/metal–oxide–semiconductor field effect transistor (MOSFET) structure that is sensitive to the presence of a molecular monolayer on its surface. The device is fabricated from a silicon-on-insulator substrate, and unlike a conventional MOSFET a substrate voltage is used to invert the buried Si:SiO2 interface. This allows the top surface of the silicon to be free of any insulating layers, apart from a thin native oxide that forms on exposure to air. The buried inversion layer is less than 40 nm away from the exposed surface, and the threshold voltage of the device is strongly influenced by the surface potential. Measurements of the drain current as a function of substrate voltage can be accurately reproduced from numerical simulation by treating the charge at the native oxide interface as a fitting parameter. The shift in threshold voltage after molecular attachment can be accounted for by a simple increase in the (positive) fixed oxide charge density, all of the other parameters being kept constant. This suggests that the shift in threshold voltage occurring after attachment of the molecular monolayer results from protonation of the native oxide. We explain this result in terms of the higher acidity of the molecular group compared to that of the native oxide.