Extended-Gate pH Sensors Using Self-Aligned Four-Terminal Metal Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors on Glass Substrate
Authors: Ohsawa Hiroki , Hara Akito 
Published: February 2020

High-performance polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) on glass substrate are applicable to high-performance and low-cost pH sensors. We previously reported on four-terminal (4T) self-aligned planar embedded metal double-gate (E-MeDG) low-temperature (LT) poly-Si TFTs on glass substrate. This device includes a metal top gate (TG) and a metal bottom gate (BG), which are fabricated from a sputtered tungsten film. The BG is embedded in the glass substrate through chemical mechanical polishing, and the TG is fabricated in a self-aligned manner to the BG by using back-surface exposure. The material of the gate dielectric is silicon dioxide (SiO2), which is deposited using plasma-enhanced chemical vapor deposition, and has a thickness of 75 and 150 nm against the TG and BG, respectively. The material of the channel region is poly-Si, which is fabricated using diode-pumped solid-state continuous-wave laser lateral crystallization. The TG and BG can be used as the drive and control gates, and vice versa.
In this device, we were able to control the threshold voltage (Vth) by varying the voltage applied to the control gate, and we confirmed a high Vth controllability. This Vth controllability is applicable for pH sensors because voltage variation applied to the control gate due to a change in pH can be read as the Vthvariation of the TFTs. Therefore, the 4T self-aligned planar E-MeDG LT poly-Si TFT can be used as an extended-gate field-effect transistor (EGFET) to measure the voltage variation due to a change in pH. Thus, we performed an evaluation of pH sensors by using the 4T self-aligned planar E-MeDG LT poly-Si TFT as an EGFET.
The evaluation of pH sensors was performed using a typical glass electrode, reference electrode, and pH buffer solution in addition to the 4T self-aligned planar E-MeDG LT poly-Si TFTs. The glass electrode was connected to the TG or BG of the TFTs. The reference electrode was connected to the electrode, which applied a constant voltage. The glass electrode and reference electrode were set in the pH buffer solution. In this situation, we evaluated the pH sensors by measuring the transfer characteristics of the TFTs by using another gate, which was not connected to the glass electrode, as the drive gate.
In the first experiment, we connected the glass electrode to the BG and measured the transfer characteristics by using the TG as the drive gate against pH = 8.8 and pH = 4.2 buffer solutions under various reference electrode voltages. We confirmed that the transfer characteristics of the drive gate in the pH = 8.8 buffer solution shifted toward the positive direction against the characteristics in the pH = 4.2 buffer solution at every reference electrode voltage. The pH sensitivity calculated from the Vthvariation is 21.7 mV/pH, which is fairly consistent with the theoretical pH sensitivity of 25.5 mV/pH.
In the second experiment, we connected the glass electrode to the TG and measured the transfer characteristics by using the BG as the drive gate against pH = 8.7 and pH = 4.8 buffer solutions at various reference electrode voltages. We confirmed that the transfer characteristics of the drive gate in the pH = 8.7 buffer solution shifted toward the positive direction against the characteristics in the pH = 4.8 buffer solution. This trend is the same as that of the top-gate drive TFT, but the pH sensitivity calculated from the Vthvariation is 76.9 mV/pH, which is larger than that of the top-gate drive TFT. In addition, its magnitude is found to be fairly consistent with the theoretical pH sensitivity of 88.8 mV/pH.
We consider that the difference in pH sensitivity between the TG and BG drives is caused by the difference in thickness between the TG and BG SiO2 dielectric. We previously confirmed that the ratio of the Vthvariation against the control gate voltage variation is larger in the BG drive than in the TG drive. We consider that the difference in pH sensitivity reflects this phenomenon. Thus, the experimental results indicate that 4T self-aligned planar E-MeDG LT poly-Si TFTs can be successfully operated as pH sensors.
In summary, we confirmed that 4T self-aligned planar E-MeDG LT poly-Si TFTs can be successfully operated as pH sensors. In addition, to improve the pH sensitivity of EGFETs, our experimental results indicate that the combination of the capacitance of the top and bottom gates is a significant parameter.