Simple Fabrication Method for Solution-gated One-piece Transistors for Biosensing Applications
Authors: Katayama Ritsu , Sakata Toshiya 
Published: September 2023

INTRODUCTION
Solution-gated field-effect transistors (FETs) with silicon or other semiconductive materials as channels can specifically and selectively detect ions and biomolecules related to biological functions by chemically modifying the gate electrode surface, and these are known as biologically coupled FETs (Bio-FETs) [1]. Since Bio-FETs can directly detect the charges of ions and biomolecules, they do not require to label fluorescent dyes and to induce redox reactions based on enzymes. Therefore, Bio-FETs are expected to be used as in vitro diagnostic devices for a label-free monitoring of health conditions in daily life.
Recently, indium tin oxide (ITO) has been widely used as an electrode material for flat panel displays owing to its transparency and conductivity. Our laboratory previously reported that solution-gate FETs fabricated by one-step sputtering provided a steep subthreshold slope (SS) owing to the absence of the interfaces among source/channel/drain electrodes and the large electric double-layer capacitance caused by direct contact between electrolyte solutions and the channel surface [2]. While this technique was simple, it utilized the diffusion of atoms during sputtering between a modified mask and substrates and was also dependent on the mask size, which limited the miniaturization of the transistor. On the other hand, the conventional fabrication methods of solution-gated FETs facilitate the miniaturization and the precise control of device structure by use of photolithography, but such methods inevitably produce impurities at the source/channel/drain electrode interfaces and may cause the degradation of device performances.
In this study, we propose a new method to fabricate solution-gated ITO thin-film channel FETs without interfaces among the source/channel/drain electrodes, which facilitates to miniaturize and control the device structure. A conductive ITO thin film with a thickness of about 100 nm is deposited on a glass substrate by sputtering. Then, the ITO film is protected by photoresist and then the channel area is exposed by photolithography. The exposed area is etched to 20 nm or less thickness, which induces semiconductive characteristics. This method enables the fabrication of the desired solution-gated ITO thin-film channel FET. Furthermore, we evaluate its electrical characteristics and examines its potential for biosensor applications.
EXPERIMENTAL
First, photoresist was patterned on a glass substrate by photolithography. ITO thin film (100 nm thickness) was deposited on the patterned area by sputtering, and the photoresist was removed using a solvent. Next, photolithography was used again to deposit the resist film so that the channel area was exposed. Etching was performed by dropping 0.1 M hydrochloric acid on the exposed channel area (Figure). Etching time was controlled by applying a voltage between the source and drain electrodes and measuring the change in current over time. A semiconductor parameter analyzer was used to measure the current during etching, the gate voltage (VG)-drain current (ID) transfer characteristics, and the pH responsivity of the fabricated devices. The film thickness was measured by a white interferometer-equipped laser microscope (Keyence).
RESULTS and DISCUSSION
By controlling the current during etching, the current value was decreased rapidly at the end of the etching. This may be because the thickness of ITO film was thinner and reached about 20 nm, which resulted in the semiconductive characteristics. Then, the etching was stopped to obtain the solution-gated ITO thin-film channel FET. The fundamental electrical characteristics such as VG-ID transfer curves were investigated, and the fabricated device was found to operate as the solution-gated FET. It was also found that the devices showed the Nernst response for the change in pH and a steep subthreshold slope of approximately 90 mV/decade (pH 7.41). Furthermore, the ITO channel thickness was measured by laser microscopy; as a result, it was about 10–20 nm.
CONCLUSIONS
In this study, we proposed the new method to fabricate the solution-gated ITO thin-film channel FET. The device actually worked as a transistor, and it showed good sensitivity to potential. This method enabled the solution-gated FETs to be produced by the simple method. Thus, the solution-gated ITO thin-film channel FET is applied to biosensors, which is highly sensitive and affordable to detect biomolecules in the future.
REFERENCES
[1] Sakata, T. ACS Omega, 4, 11852 (2019).
[2] Sakata, T.; Nishitani, S.; Saito, A.; Fukasawa, Y. ACS Appl. Mater. Interfaces,
13, 38569 (2021).

Figure 1