| For over 30 years, field effect transistors (FETs) have been used as ion sensors. Recently, functionalized silicon nanowires have been introduced into sensor architecture to detect a variety of species including proteins, DNA, and even single viruses. The application to a biological environment introduces new challenges. For instance, in addition to specific device designs, careful consideration must be given to the properties of the liquid media and its interactions with the FET. This is paramount not only to the resolution of the "true" sensing signal from that of other factors but also to enhancing manufacturability and reliability of these devices.;This work focuses on a variety of issues that impact the electrical properties of Silicon-on-insulator FET-based sensor devices. Biological solutions consist of protein or DNA in an electrolytic solution containing salt ions. Some of these ions, such as Na, have long been known to cause instabilities in MOS devices. The effect of mobile ions on SOI-based sensors is studied. Na is shown to cause permanent hysteresis in the devices while K is not. Devices with the gate oxide protected by self-assembled monolayers (SAMs) do not show hysteresis.;Additionally, the SOI-based sensor structure results in the electrolyte voltage being capacitively coupled to the back gate voltage. Coupling causes the top channel of the device to be modulated by the electrolyte voltage. The electrolyte voltage however, couples non-linearly and lower coupling ratios are observed during I-V measurements compared to I-t measurements. Hysteresis is observed in the measured electrolyte voltage between the forward and reverse sweeps of the back-gate voltage. Hysteresis is also observed in the I-V curves of the sensor device in the presence of electrolytic solutions. I-V hysteresis is strongly dependent on the electrolyte voltage hysteresis. 1D models developed by Dr. Dick Chapman based on a Poisson solver with site-binding theory corroborate the observed device characteristics -- low threshold voltages and good sub-threshold slopes Finally, physically realistic SPICE models were developed and illustrate the response of both pH and biosensors with a multi-gate model. The model demonstrates good agreement to experimental data. It also shows that logarithmic increments in bound membrane (protein/DNA) charge should result in linear threshold voltage shifts. The model accounts for phenomena such as Debye screening of biomolecules by salt ions. Screening effects have been shown to reduce the response of a sensor. Additionally, the presence of site binding charge on a SiO2 surface severely deteriorates sensitivity. |
