Shigeru Aomori

Threshold voltage control of bottom-contact n-channel organic thin-film transistors using modified drain/source electrodes

Bottom-contact n-channel C60 thin-film transistors (TFTs) with drain/source electrodes modified by benzenethiol derivatives have been fabricated to investigate the influence of the modification on the transistor characteristics. Modification using methylbenzenethiol, aminobenzenethiol, and (dimethylamino)benzenethiol having electron-donating groups causes threshold voltages to shift to low voltages. In addition, the modification provides no significant decrease in saturation mobilities. A C60 TFT with (dimethylamino)benzenethiol-modified electrodes has a low threshold voltage of 5.1 V as compared to that of 16.8 V for a TFT with nonmodified electrodes. The threshold-voltage shift is probably because the modification reduces electron-injection barrier height and improves electron injection into organic semiconductors.

Bottom-contact fullerene C60 thin-film transistors with high field-effect mobilities

Fullerene C60 thin-film transistors (TFTs) with bottom-contact structure have been fabricated. The parasitic resistance was extracted using gated-transmission line method. The drain/source electrodes consisted of a single Au layer with no adhesion layer; the channel lengths ranged from 5 to 40 μm. The field-effect mobilities in the saturation regime slightly depended on the channel length, ranging from 2.45 to 3.23 cm2/V s. The mobility of 3.23 cm2/V s was obtained from the TFT with a channel length of 5 μm and is the highest in organic TFTs with bottom-contact structure. The high mobility is due to the low parasitic resistance.

High conductance bottom-contact pentacene thin-film transistors with gold-nickel adhesion layers

Bottom-contact pentacene thin-film transistors (TFTs) with different electrode types have been fabricated to investigate influence of the adhesion layers and surface modification on the transistor characteristics. Gold-nickel alloy or Ti was used for the adhesion layer; the drain/source electrodes were modified with pentafluorobenzenethiol (PFBT) or nonmodified. The TFTs with AuNi layers and PFBT-modified electrodes exhibit channel-length independent saturation mobility. The electrode-type TFT with a channel length of 2.2 μm has a saturation mobility of 0.73 cm2/V s and a transconductance of 229 μS/mm. The high performance is attributed to the low contact resistance of 408 Ω cm.

High-performance fullerene C60 thin-film transistors operating at low voltages

Low-voltage operation of fullerene C60 thin-film transistors (TFTs) has been realized using zirconium-silicon oxide (ZSO) as a gate insulator. The gate insulator consisted of triple layers of SiO2∕ZSO∕SiO2 deposited by rf sputtering. The C60 TFTs with the insulators operated at a low voltage of 5V. The surface of the insulator was treated with hexamethyldisilazane (HMDS) or octadecyltrimethoxysilane (ODS). The C60 TFT with the ODS-treated insulator exhibited higher performance than that with the HMDS-treated insulator, and it had a field-effect mobility of 1.46cm2∕Vs, threshold voltage of 1.9V, and a current on/off ratio of 2×106.

Solvent Dependence of Vacuum-Dried C60 Thin-Film Transistors

We demonstrated solution-processed C60 thin-film transistors with high electron mobility. C60 solutions in various organic solvents were dried in a vacuum chamber to obtain uniform thin films. While C60 solution dried under atmospheric pressure produced a large number of crystals, vacuum-dried C60 solution provided flat and uniform thin films of sufficiently high quality to fabricate thin-film transistors. In spite of amorphous-like thin-film formation, C60 transistors showed strong solvent dependence. High performance C60 thin-film transistors with field-effect mobility of 0.86 cm2 V-1 s-1, threshold voltage of 1.5 V, subthreshold slope of 0.67 V/decade and a current on/off ratio of 3.9 ×106 were obtained from 1,2,4-trichlorobenzene C60 solution.