Amorphous Indium-Gallium-Zinc-Oxide Transistor Used Ion Gel Dielectric for Low Driving Voltage and Easy Processing
Authors: Kim Jinkyoo , Park Sang-Hee Ko 
Published: February 2020

Transistors using organic materials are attracting huge attention in the market today due to their low cost and possibility of solution process. Moreover, since the organic materials used in transistors are intrinsically flexible, they are believed to be essential elements for the realization of flexible devices such as stretchable displays, rollable displays, and wearable devices. In most cases, however, because of the low dielectric constant of organic materials, the operating voltage of the transistor is very high when the organic materials are used as the gate dielectric layer.
When the operating voltage of the transistor is high, two serious problems can occur. First, the high operating voltage can cause increased power consumption of the transistor. It is a fatal drawback that reduces the applicability to wireless device applications because in the case of wireless electronic equipment, lifetime of battery is limited. Also, as the operating voltage increases, additional gate driver integrated circuits are required to control the driving voltage. This leads to the increased process complexity, and reduces the price competitiveness of the device. Therefore, it can be said that the possibility of mass production and functionality of transistors using organic materials will increase by solving these two problems. As a result, various studies have been focused on solving these problems by reducing the thickness of dielectric or choosing unique organic material with high dielectric constant [2] [3].
In order to solve above problems, we proposed the transistors based on ion gel dielectric, which has high capacitance. Ion gel refers to the substance in which ionic liquid is trapped in the polymer matrix [1]. When the voltage is applied to the electrode, the ion gel forms an electric double layer (EDL) at the interface with the electrode. Since EDL forms a capacitor with thickness of several nanometers, the ion gel has a very high capacitance above 1μF/cm2 [1].
From the capacitance-frequency curve measurement of Metal-Ion gel insulator-Metal capacitor (MIM), it was confirmed that the capacitance of ion gel increased as the amount of ionic liquid in ion gel increased. In addition, MIM showed the capacitance around 1μF/cm2 even if the ionic liquid content was very small. In the case of the transistor structure, the transistor was easy to fabricate because the gate electrode was deposited on the same substrate as the source-drain electrode. The active layer had the width of 20μm and the length of 160μm. The gate leakage current of the transistor increased with increasing the amount of ionic liquid in the ion gel. By optimizing the amount of ionic liquid to control gate leakage current, the ionic liquid was chosen 3% relative to the polymer in terms of mass and the gate leakage current was reduced to several tens of picoampere. The transfer characteristic represented that turn-on-voltage was near -1V and on-off ratio was 3.93·104 when the gate voltage was swept from -4V to 4V. This result shows that the transistor can operate with well-behaved electrical property even at small driving voltage near 4V.
The transistor made by simple fabrication with ion gel dielectric showed outstanding electrical characteristics with low driving voltage around 4V and this result will pave the way for the next generation of flexible devices.
[1] Wang, H., Wang, Z., Yang, J., Xu, C., Zhang, Q., & Peng, Z. (2018). Ionic Gels and Their Applications in Stretchable Electronics. Macromolecular rapid communications, 39(16), 1800246.
[2] Yoon, M. H., Yan, H., Facchetti, A., & Marks, T. J. (2005). Low-voltage organic field-effect transistors and inverters enabled by ultrathin cross-linked polymers as gate dielectrics. Journal of the American Chemical Society, 127(29), 10388-10395.
[3] Hung, C. C., Wu, H. C., Chiu, Y. C., Tung, S. H., & Chen, W. C. (2016). Crosslinkable high dielectric constant polymer dielectrics for low voltage organic field‐effect transistor memory devices. Journal of Polymer Science Part A: Polymer Chemistry, 54(19), 3224-3236.
This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609)

Figure 1