Novel silicon metal-oxide semiconductor devices for molecular sensing and hot electron spectroscopy

This dissertation describes results from a novel hybrid molecular/metal-oxide-semiconductor field effect transistor (MOSFET). The device consists of a buried channel silicon-on-insulator MOSFET with a molecular monolayer attached to its surface. The device is made using a 2 μm minimum gate length, silicon MOSFET process developed in Arizona State University. A hybrid molecular/MOSFET structure that is sensitive to the presence of a molecular monolayer on its surface was fabricated and characterized. The device was fabricated from a silicon-on-insulator (SOI) substrate. A substrate voltage was used to invert the buried Si:SiO ₂ interface in this device. This allowed 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 was less than 40 nm away from the exposed surface, and the threshold voltage of the device was 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. A large shift in the threshold voltage occurring after attachment of the molecular monolayer was observed, which was explained by molecular protonation of the native oxide. A split gate SOI device structure was also developed and fabricated to improve the sensitivity of the hybrid molecular/MOSFET. Because of its very small size, the nano-scale hybrid device will be susceptible to hot carrier effects. Numerical simulation of a 100nm x 100nm split gate device showed high electron velocity at the drain end of the channel. A novel scanning probe technique is suggested as a way of measuring the hot electron distribution in the channel of the split gate structure. This structure is also suitable for hot electron spectroscopy in ultra short channel MOSFETs because it resembles a conventional MOSFET and its inversion channel is close to the ambient and therefore accessible with the tip of a scanning probe microscope.