Solution-Gated One-Piece ITO-Based 2D Transistor for Biosensing Applications

Solution-gated field-effect transistors (FETs) have attracted attention owing to their applicability to biological sensing. Various semiconductive materials have been widely utilized as the channel of FETs for biosensing devices, such as one-dimensional (1D) and two-dimensional (2D) materials. When the channel thickness of 2D materials is smaller than the maximum depletion width, which is more than the extrinsic Debye length, that is, the screening length of Coulomb scattering, the electrical communication between the source and drain (S/D) electrodes is induced by a very small density of free electrons in the depletion region, resulting in a clear on/off state of FETs. Thus, 2D-material FETs, the channel thickness of which is controlled to be approximately less than the depletion width, are expected to be highly sensitive in detecting of biomolecules that attach to the channel in a solution. That is, a solution-gated 2D-channel FET biosensor, the channel of which is directly in contact with an electrolyte solution, is expected to have a steep subthreshold slope (SS), resulting in an ultrahighly sensitive biosensing owing to a relatively large capacitance of the electric double layer at the electrolyte solution/channel interface.<br/>Among such semiconductive materials, we focus on a thin-film indium tin oxide (ITO) with a thickness less than 30 nm, which is useful as the channel of a solution-gated 2D-channel FET biosensor [1–3]. In particular, the solution-gated ITO-based FET sensor with the ultrathin channel was fabricated by a one-step procedure, which contributed to the all-by-ITO technology [1]. This was simply realized by using the metal shadow mask with thin sheets at both ends because the bottom of the metal shadow mask corresponding to the channel became slightly raised from the substrate owing to the thin sheets and then some particles crept into the gap during sputtering. The 20-nm-thick ITO channel resulted in the bandgap increase and free-carrier depletion in the same way as in 2D materials. The threshold thickness of the depleted ITO channel was determined to be approximately 30 nm from the significant change in the on and off current ratio. The remarkably steep SS (ca. 60–80 mV/decade) was derived from the interfacial trap-free structure based on the one-step fabrication, which was due to the absence of the interfaces among source/channel/drain ITO electrodes “One-Piece ITO-based 2D Transistor”, as well as the relatively large capacitance of C_dl based on the direct contact of electrolyte solutions with the ITO channel, and caused the exponential pH sensitivity in the subthreshold regime. This one-piece ITO-based 2D transistor could be fabricated by an etching method of 100 nm-thick conductive ITO to 20 nm-thick semiconductive ITO channel [2]. Moreover, in this talk, we would like to show the ultrasensitive DNA detection as a model of biomolecular recognition in the subthreshold regime of the one-piece ITO-based 2D transistor [3] and then mention its perspective.<br/><b>References</b><br/>[1] Sakata, T.; Nishitani, S.; Saito, A.; Fukasawa, Y. <i>ACS Appl. Mater. Interfaces</i> 2021, 13, 28569–38578. [2] Katayama, R.; Sakata, T. <i>ECS Trans.</i>2023, 111, 37–43. [3] Katayama, R.; Sakata, T. <i>under review</i>.