Wide-Range Resistance Modulation on a SmNiO3 Chemical Transistor
Authors: Hattori Azusa N. , Kawamoto Daiki , Yamamoto Mahito , Tanaka Hidekazu 
Published: February 2020

A SmNiO3 (SNO) chemical field effect transistor (FET) using ion liquid as gate insulator shows large and non-linier resistance change depending on Vg application condition. The resistance modulation corresponds to redox reaction at the interface between ionic liquid and SNO channel (Ni3+→Ni2++hole), however, the quantitative understanding for SNO chemical transistor operation is still lacking. We systematically studied the correlation between Ni valence state and modulated resistance ratio. The channel resistance of the SNO chemical FET changed nonlinearly over a wide range for different temperatures, Vg magnitudes, and Vg application durations. The correlation between the modulated resistance and the Ni valence state was quantitatively revealed using X-ray photoelectron spectroscopy (XPS). A model describing the modulated resistance value (Rmod) with the Vg application conditions: operation temperature (T), a magnitude of gate voltage (Vg) and Vg application duration (t), was proposed in Eq. (1) by considering the kinetics of the reduction reaction on the SNO channel.
Rmod=exp[A∙exp[B∙Vg]∙t]----Eq.(1)
The parameter A is the Ni2+ creation speed without Vg application, which is experimentally obtained. The parameter B corresponds to the acceleration parameter and is temperature-dependent. Equation (1) allows us to calculate the time development of the resistance value under various Vg application conditions. Figure (a) shows the modulated resistance evaluated as a function of the Vg magnitude and duration time. With increasing Vg and t, the resistance nonlinearly increases. One can see that our proposed model enables the resistance to be predicted for given Vg application conditions and selective resistance modulation over a wide range of resistances has been demonstrated. Figure (b) shows a comparison between the experimental time evolution of the modulated resistance (blue circles) and values calculated from eq. 1 (green triangles) under various Vg application conditions (lower panel). The calculation of the modulated resistance shows good agreement with the experimental values at Vg of 2.6 V (left) and 3.2 V (middle). However, at Vg = 3.6 V (right), the experimentally obtained resistance value (∼104) was 2 orders smaller than the calculated value (∼106). This difference seems to be caused by the simplified model, where the Ni2+ creation speed is assumed to be constant during Vg application. At the actual SNO surface, the speed of Ni2+ creation (Ni3+ consumption) may decrease with the passage of time because of the exhaustion of Ni3+ on the SNO surface. The discrepancy between the experimental and calculated values should increase with Vg. Additionally, at a Vg of more than 3.0 V, the leak current (the current between the gate and drain electrodes) became large, suppressing the operation of the FET [1].
The model was able to quantitatively describe the resistance modulation behavior in the SmNiO3 chemical FET, and wide range resistance control was demonstrated. The control of a chemical FET with flexible and nonlinear responses, which are absent in a conventional FET, is expected to contribute to the generation of new properties, such as biomimetic switches and/or sensors behaving similarly to the human brain. In the presentation, the improvement of resistance modulation efficiency by the channel size reduction on the SMNiO3 chemical transistors will be shown.
[1] D. Kawamoto et al., ACS Appl. Electron. Mater., 1 (2019) 82-87.

Figure 1