Steep Subthreshold Slope in Solution-Gated Indium–Tin–Oxide-Based One-Piece Thin-Film Transistor Enables Highly Sensitive Biosensing

Solution-gated two-dimensional material-like thin-film transistors (2D-like TFTs) have made it possible to realize miniaturized and portable biosensors as electronic devices. As one of the transistor characteristics, a steep subthreshold slope (SS) must increase the detection sensitivity to target biomolecules because a slight change in gate potential is translated into a large rate of change in drain current (IDS). The 2D-like TFT biosensor we developed, a solution-gated one-piece indium–tin–oxide-based TFT (one-piece ITO-TFT), intrinsically shows a steep SS near the thermal limit (60 mV/decade) owing to the absence of interfaces in the source/channel/drain electrodes and the relatively large electrical double-layer capacitance at the electrolyte solution/ITO channel interface. In this study, we have demonstrated the increase in detection sensitivity on the basis of the rate of change in IDS measured in the subthreshold region using the one-piece ITO-TFT with a steep SS. The detection sensitivity to DNA hybridization as a model of biomolecular recognition events measured in the subthreshold region was about 6.6 times higher than that measured in the linear region, as expected from the theoretical assumption. Moreover, the limit of detection (LOD) was almost at the same concentration even when measured in either the subthreshold or linear region. This is because LOD strongly depended on the binding constant (Ka) of DNA hybridization rather than the detection sensitivity. Considering the above, the IDS measurement in the subthreshold region using transistors with a steep SS definitely contributes to the increase in detection sensitivity, whereas LOD would be based on Ka regardless of the detection region.