Abstract
Field-effect transistor (FET)-based biosensors have garnered significant attention for their label-free electrical detection of charged biomolecules. Whereas conventional output parameters such as threshold voltage and channel current have been widely used for the detection and quantitation of analytes of interest, they require bulky instruments and specialized readout circuits, which often limit point-of-care testing applications. In this study, we demonstrate a simple conversion method that transforms the surface potential into an oscillating signal as an output of the FET-based biosensor. The oscillation frequency is proposed as a parameter for FET-based biosensors owing to its intrinsic advantages of simple and compact implementation of readout circuits as well as high compatibility with neuromorphic applications. An extended-gate biosensor comprising an Al2O3-deposited sensing electrode and a readout transistor is connected to a ring oscillator that generates surface potential-controlled oscillation for pH sensing. Electrical measurement of the oscillation frequency as a function of pH reveals that the oscillation frequency can be used as a sensitive and reliable output parameter in FET-based biosensors for the detection of chemical and biological species. We confirmed that the oscillation frequency is directly correlated with the threshold voltage. For signal amplification, the effects of circuit parameters on pH sensitivity are investigated using different methods, including electrical measurements, analytical calculations, and circuit simulations. An Arduino board to measure the oscillation frequency is integrated with the proposed sensor to enable portable and real-time pH measurement for point-of-care testing applications.
Highlights
Field-effect transistor (FET)-based biosensors have received significant attention for their label-free electrical detection of chemical and biological species
We demonstrated a conversion method that enables surface potential-controlled oscillations to provide the oscillation frequency as an output parameter for FET-based biosensors
The ring oscillator connected to the extended-gate biosensor can convert the pH-induced surface potential generated in the Al2O3 layer of the extended gate into an oscillation signal
Summary
Field-effect transistor (FET)-based biosensors have received significant attention for their label-free electrical detection of chemical and biological species. Because of their inherent similarity with conventional FETs in terms of device structure and fabrication process, FET-based biosensors offer the well-known advantages of modern electronic devices such as high-density integration of sensor arrays in a compact size, reliable fabrication with high reproducibility, and low-cost mass fabrication. An FET-based biosensor transduces a binding event occurring on the gate surface into a change in the electrical characteristics, such as the threshold voltage or channel current [23,24,25]. The threshold voltage and channel current have been widely used as output parameters for FET-based biosensors. We present the future prospects of using the oscillation frequency in portable sensing systems based on real-time pH measurements using an Arduino board operated via open source codes
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