2, Osaka University, Osaka, , Japan
3, Advanced Photonics and Biosensing Open Innovation Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Suita, Osaka, Japan, Osaka, , Japan
In this study, we have realized threshold-voltage (Vth) controls by the selection of gate metals or the gate electrode modification with self-assembled monolayers (SAM) in organic transistors. The difference of work function between the gate metal and the organic semiconductor intrinsically varies the Vth. In other words, transistors with desired Vth can be fabricated by the modification of the gate electrodes. Using the gate modification, enhance-mode and depletion-like transistors have been realized for Al gate electrodes and SAM-treated Au gate electrodes, respectively. As a result, the Vth can be shifted for 0.95 V in 2-V-operating organic transistors, which is useful for designing organic logic circuits.
Recently, organic thin-film transistors (OTFTs) have attracted attention because of their low costs and simple processability. The technology of OTFTs is desired to be used for sensor applications in the IoT society, where the signal addressing and processing circuits are important. In such analog circuits, both low-voltage operation and highly-reliable electric characteristics are essential. Therefore, the precision and reproducible control of Vth is one of key technologies for organic analog circuits. It has been already shown by several groups that Vth can be shifted by modifying the surface of gate dielectrics with SAM or plasma treatment1,2. Although these methods can shift Vth largely, it has been realized only on the specific inorganic dielectrics, such as silicon dioxide or aluminum oxide. Another approach for the Vth control is the modification of gate metals. This is a universal method to shift the Vth because large variety of gate metals and dielectrics are applicable.
In this study, we fabricated OTFTs with modified gate electrodes, where the metal surface is treated with polarized SAM so that the work function of the metal is changed. Various kinds of gate metals and SAM modifications are investigated in bottom-gate and top-contact transistors. A thin parylene, DNTT and Au top-contact electrodes were used for the other components in the OTFTs. As a result, we realized enhance-mode OTFTs with Vth = - 1.05V for Al gate electrodes. On the other hand, we realized depletion-like OTFTs with SAM treated Ag or Au gate electrodes. We found that SAM treatment with pentachlorobenzenthiol changes the Vth for up to 140 mV. It should be noted that only the Vth can be shifted without changing the other parameters such as mobility. These Vth control technology is useful for designing organic circuits, and we will demonstrate improved circuits designs for some analog circuits such as inverters and Pseudo CMOS technologies.
1. S. Kobayashi, et al. Nature Mater. 3, 317-322 (2004)
2. A. Kitani, et al. J. Appl. Phys. 55, 03DC03 (2016)