Effects of\nSurface Oxygen Vacancies and Hydroxy Groups\non Electrical Characteristics in Solution-Gated One-Piece Indium–Tin-Oxide-Based\nField-Effect Transistors

A solution-gated indium–tin-oxide\n(ITO)-based\nfield effect\ntransistor (FET) without interfaces among the source, channel, and\ndrain electrodes, which is called the one-piece ITO-FET, can be simply\nfabricated from a single sheet of ITO by etching the channel region.\nThe direct contact of the ITO channel surface with a sample solution\ncontributes to a steep subthreshold slope and a high on/off ratio.\nIn this study, we have examined the effects of oxygen vacancies and\nhydroxy groups at the ITO channel surface on the electrical characteristics\nof the one-piece ITO-FET. In the transfer characteristics, the turn-on\nvoltage gradually increased over the days of storage in deionized\nwater and then recovered after the heat treatment at 200 °C in\nvacuum. This resulted from the finding that the densities of oxygen\nvacancies as a source of carriers and hydroxy groups as charged species\nat the ITO channel surface changed under each condition, which were\nanalyzed by X-ray photoelectron spectroscopy. In particular, the pH\nresponsivity of the one-piece ITO-FET was improved and maintained\nwith increasing the density of hydroxy groups at the ITO channel surface.\nTherefore, the control of the densities of oxygen vacancies and hydroxy\ngroups at the ITO channel contributes to the improvement of the detection\nstability of devices when targets are actually measured.