Ph.D Thesis

Ph.D StudentPaska Yair
SubjectUnderstanding the Sensing Mechanism of Nonpolar Analytes
with Field Effect Transistors
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor PROF. Hossam Haick
Full Thesis textFull thesis text - English Version


Silicon nanowire field effect transistors (Si NW FETs) have been used as powerful sensors for direct detection of biological and chemical species. Si NW FETs signal transduction is associated with moderately selective recognition of chemical and biological species. For example, oxide-coated Si NW FETs were functionalized with amino siloxanes to impart relatively high sensitivity toward pH and with a variety of biological receptors to impart selectivity toward biological species in solution. Similar approaches were used for achieving highly sensitive detection of polar analytes in the gas phase such as N2O, NO, CO, etc.  However, the detection of nonpolar analytes still remains challenging and hence not well understood to date.

In this study, we experimentally study the detection of nonpolar species using molecularly modified Si NW FETs and model the detection process based on changes in the carrier mobility, voltage threshold, off-current, off-voltage, and subthreshold swing of the Si NW FET. We compare the nonpolar result with the detection of polar species. We show that elimination of the free hydroxyl groups (Si-OH) located on the oxide surface of the Si NW, by controlled tricholorosilane (TS) adsorption process, improves both the electrical and sensing properties of the gated Si NW FET devices. The improvements are expressed through a decrease in the hysteresis magnitude and hysteresis drift of the gated Si NW sensors, through a dramatic decrease in the sensitivity to polar analytes and humidity, and through significant increase in the sensitivity to nonpolar analytes. The sensing mechanism of nonpolar analytes was explained in terms of molecular gating, due to two indirect effects: (i) a change in the charged surface states induced from conformational changes at the functionality of the Si NW surface because of analyte-functionality interactions; and (ii) a change in the dielectric medium close to the Si NW surface induced from the formation of condensed nonpolar analyte layer. In contrast, polar analytes are sensed directly via analyte-induced changes in the Si NW charge carriers, most probably due to electrostatic interaction between the Si NW and the polar analytes. The contribution of these effects to the overall measured sensing signal was determined and discussed. Semi-empirical physical models for the analyte-induced conductivity change in the Si NW FETs and for the fundamental feature of Si NW FET characteristic are presented and discussed. The results provide a launching pad for real-world sensing applications, such as environmental monitoring, homeland security, food quality control, and medicine.