Fabrication and Optimization of Organic FET and Organic Electrolyte Gated FET Devices for Applications as Label-free Biosensors

Abstract

Organic Thin-Film Transistors are carbon-based semiconductor devices. Their simple fabrication makes them an ideal candidate for disposable, wearable technologies; an implementation being aptamer-based biosensors. This project produces organic semiconductor aptamer-based biosensors for quantifying biomolecules in synthetic samples mimicking bio-fluids. There are two main effects in a field effect transistor; capacitance and resistance. The semiconductor between the source-drain electrodes is a transient resistor. Capacitance is created by voltage differences across the gate dielectric insulator. Charge buildup at the semiconductor-insulator interface induces a conducting channel through the semiconductor. Channel current can be increased by modulating the source-gate voltage until the current saturation point is reached. The transconductance, ratio of voltage to current change, is device-specific and based on resistance and capacitance. Electrolyte media incorporated within the OFET framework as part of the gate dielectric permits label-free analyte sensing. The liquid-sample/solid-dielectric interface has an electric double layer capacitive effect that alters with biomolecule concentration change, causing the OFETs to experience transconductance change. These sensor devices were fabricated using simple, low-temperature processes. The semiconductor and dielectric stacks were spun on the source-drain electrodes to form the bottom surface, and the top gate was an in-house designed conductive polymer bilayer with a thin PMMA layer then functionalized with aptamers, DNA oligomers that specifically interact with a certain biomolecule. This produces label-free sensors with highly correlated current modulation due to changes in device transconductance. The characterized devices were used to create current vs concentration curves for biomolecule solutions of known concentration. The generated response curves were used to directly correlate device output current to biomolecule concentration. The extensive material and processing optimization work conducted in the thesis yielded several performance outcomes comparable to high fabrication requirement OFETS, such as charge mobilities of ~ 0.1 – 0.2 cm2/V-s. The OFET on/off ratios is 1.3x104. The OEGFET device detects 1-2 orders of magnitude change in dosage with a limit of detection of 0.01 µM. An OFET device optimized with a novel dielectric stack demonstrated significant improvements in operating parameters and shelf life.