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.
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.
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.
Organic complementary circuits consisting of bottom-contact p-channel pentacene and n-channel C60 thin-film transistors (TFTs) have been fabricated to evaluate their dynamic properties. Modified drain and source electrodes were used to balance the threshold voltages of the pentacene and C60 TFTs. The balanced threshold voltage allowed use of equal-size channel dimensions for both channel-type TFTs in the circuits. The signal delay per stage of a five-stage ring oscillator was consistent with the mobilities of the individual TFTs. The oscillation frequency increased with supply voltage up to 200 kHz, which is the highest frequency in organic five-stage complementary ring oscillators.
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