Understanding and Optimization of Field-Effect Transistor-Based Biological and Chemical Sensors
Over the past 15 years, biologically sensitive field-effect-transistors (bioFETs) employing a nanowire as their active element have become very popular. Each year, many experiments are reported that use nanowire bioFETs to detect and quantify new clinically relevant biomolecules.
This thesis aims to enhance our understanding of bioFETs in order to answer important practical questions regarding the operation and optimization of them.
We discuss the proper biasing of a bioFET in order to maximize sensitivity. Using electrostatic arguments and MATLAB simulations, we offer a recipe for biasing that applies to biomolecule detection using SOI-based FETs, including nanowire and nanoribbon bioFETs. In contrast to previous works, we take into account the two degree of freedom afforded by a solution-gate and a back-gate. We find that such a bioFET is best operated when the channel is fully depleted. This is in line with the findings of others regarding the biasing of bioFETs using a single gate. We also discuss the opportunity to tune an extra degree-of-freedom: the electric charge on the surface of bioFETs that could be brought by other chemical species, such as linker molecules, probe molecules, or surface groups. We show that an optimum value exists for this surface charge. The optimum amount of charge is that which minimizes the electric field at the solution/bioFET interface prior to analyte capture. We show that this optimum is a result of a minimum in Debye screening strength.
We study the electrostatics of a nanowire bioFET as we change the geometry of the nanowire. It is commonly accepted that nanowires with small diameters are more sensitive than larger nanowires because their surface-area-to-volume ratio is larger. We use analytical arguments to show that this analysis is incorrect. Using simulations, we show that analytes captured in concave regions and corners produce a stronger signal due to reduced Debye screening. All structures have such corners, but in small nanowires, these corners are a larger fraction of the entire surface, so the positive effect of the corner is more apparent, leading to higher sensitivity for smaller nanowires.
We discuss the necessity of solution electrodes in the operation of bioFETs. We show that without a solution electrode, the sample solution will capacitively couple to various FET terminals, resulting in an unpredictable bias point, and an unpredictable return path for the bioFET electric field. These effects could adversely affect the reproducibility of bioFET measurements. We use analytical arguments to show that a return path must exist for the electric fields that emanate from the biomolecules and enter the bioFET. We use simulations to show that the uncontrolled capacitive coupling between the sample solution and the bioFET terminals provides this path in the absence of a solution electrode. We also show that slight variations in the structure could affect this capacitive coupling and change the sensitivity in largely unpredictable and unexpected ways. We conclude that either a solution electrode must be present, or the sample solution must be coupled to a terminal using a controlled large capacitance in order to minimize the effect of variations. We also show that lack of a solution electrode can be a source of signal drift in experiments.