Yusaku Kato

Integration of organic FETs with organic photodiodes for a large area, flexible, and lightweight sheet image scanners

A large-area, flexible, and lightweight sheet image scanner has been successfully manufactured on a plastic film by integrating high-quality organic transistors and organic photodetectors. The effective sensing area of the integrated device is 5/spl times/5 cm/sup 2/; the resolution, 36 dots per inch (dpi); and the total number of sensor cells, 5184. The pentacene transistors with top contact geometry have a channel length of 18 /spl mu/m and mobility of 0.7 cm/sup 2//Vs. Organic photodetectors composed of copper phthalocyanine and 3,4,9,10-perylene-tetracarboxylic-diimide distinguish between black and white parts on paper based on the difference in their reflectivity. Since this new area-type image-capturing device does not require any optics or mechanical scanning devices, the present sheet image scanners are mechanically flexible, lightweight, shock resistant, and potentially inexpensive to manufacture; therefore, they are suitable for human-friendly mobile electronics.

Bending experiment on pentacene field-effect transistors on plastic films

We have fabricated very flexible pentacene field-effect transistors with polyimide gate dielectric layers on plastic films with a mobility of 0.3cm2∕Vs and an on/off ratio of 105, and have measured their electrical properties under various compressive and tensile strains while changing the bending radius of the base plastic films systematically. We have found that the change in source-drain current with bending radius is reproducible and reversible when the bending radius is above 4.6mm, which corresponds to strains of ∼1.4±0.1%. Furthermore, the change in source-drain current does not depend on the direction of strain versus direction of current flow.

High mobility of pentacene field-effect transistors with polyimide gate dielectric layers

Polyimide gate dielectric layers cured at 180 °C have been employed to fabricate high-quality pentacene field-effect transistors on polyethylenenaphthalate-based films. The surface roughness (root-mean square) of gate dielectric layers characterized by atomic force microscopy is only 0.2 nm, while that of the base film is 1 nm. The transistors with pentacene channel layers deposited on 990 nm polyimide gate dielectric layers attain the on/off ratio of 106 and mobility of 0.3 cm2/V s. Furthermore, by decreasing the thickness of polyimide gate dielectric layers down to 540 nm, the mobility is enhanced up to 1 cm2/V s.

Sheet-Type Braille Displays by Integrating Organic Field-Effect Transistors and Polymeric Actuators

A large-area, flexible, and lightweight sheet-type Braille display has been successfully fabricated on a plastic film by integrating high-quality organic transistors and soft actuators. An array of rectangular plastic actuators is mechanically processed from a perfluorinated polymer electrolyte membrane. A small semisphere, which projects upward from the rubberlike surface of the display, is attached to the tip of each rectangular actuator. The effective display size is 4times4 cm2. Each Braille letter consists of 3times2 dots and 24 letters; in other words, 6 letters times 4 lines can be displayed. Pentacene field-effect transistors with top-contact geometry have a channel length of 20 mum and a mobility of 1 cm2/Vmiddots. The Braille dots on one line are driven for 0.9 s. The total thickness and weight of the entire device are 1 mm and 5.3 g, respectively. The present scheme will enable people with visual impairments to carry the Braille sheet display in their pockets and read Braille e-books at any time. Since all the device components are manufactured on plastic films, these sheet-type Braille displays are mechanically flexible, lightweight, shock resistant, and potentially inexpensive to manufacture; therefore, they are suitable for mobile electronics

Ultraflexible organic field-effect transistors embedded at a neutral strain position

We fabricated ultraflexible pentacene field-effect transistors (FETs) with a mobility of 0.5cm2∕Vs and an on/off ratio of 105, which are functional at the bending radius less than 1mm. The transistors are manufactured on a 13-μm-thick polyimide film and covered by a 13-μm-thick poly-chloro-para-xylylene encapsulation layer so that transistors can be embedded at a neutral position. This sandwiched structure can drastically suppress strain-induced changes in transistor characteristics. Furthermore, the FETs show no significant change after bending cycles of 60 000 times on inward and outward bending stresses.

A large-area flexible wireless power transmission sheet using printed plastic MEMS switches and organic field-effect transistors

We have successfully manufactured a large-area power transmission sheet by using printing technologies. The position of electronic objects on this sheet can be contactlessly sensed by electromagnetic coupling using an organic transistor active matrix. Power is selectively fed to the objects by an electromagnetic field using a plastic MEMS-switching matrix

Control of threshold voltage of organic field-effect transistors with double-gate structures

We fabricated pentacene field-effect transistors with planar-type double-gate structures, where the top- and bottom-gate electrodes can independently apply voltage biases to channel layers. The threshold voltage of organic transistors is changed systematically in a wide range from −16to−43V when the voltage bias of the top-gate electrode is changed from 0to+60V. The mobility in the linear regime is almost constant (0.2cm2∕Vs) at various voltage biases of the top-gate electrode and the on/off ratio is 106.

Suppression of DC bias stress-induced degradation of organic field-effect transistors using postannealing effects

We fabricate pentacene field-effect transistors (FETs) showing a very small degradation in performance under a continuous DC bias stress. Pentacene FETs are manufactured on polyimide films with polyimide gate dielectric layers, and then encapsulated by poly-chloro-para-xylylene passivation layers, resulting in very flexible and heat-resistant devices. When such devices are annealed at 140°C for 12h in a nitrogen environment, the change in their source-drain current is 3±1% even after the application of continuous DC voltage biases of VDS=VGS=−40V for 11h. Furthermore, their mobility is increased by postannealing effects from 0.27cm2∕Vsto0.36cm2∕Vs and their on/off ratio is also increased from 103 to 106.

Large-Area Flexible Ultrasonic Imaging System With an Organic Transistor Active Matrix

We have successfully fabricated a large-area flexible ultrasonic imaging system by integrating a polymeric ultrasonic-transducer array sheet with an active matrix of organic field-effect transistors. The ultrasonic sheet comprises 8 x 8 ultrasonic sensing cells with an effective size of 25 × 25 cm2. The organic transistors exhibit mobility of 0.1 and 0.5 cm2/V ·s at the low operation voltage of 1 mV and in the saturation regime, respectively. When an ac signal is applied between source and drain electrodes for the transistors with a grounded gate, the on/off ratio is larger than 104 at the carrier frequency of 40 kHz. In the linear sensing array comprising eight ultrasonic cells, crosstalk is suppressed sufficiently low, and the on/off ratio exceeds 104. Images in free space are obtained for multiple-target objects over this sheet.

An Organic FET SRAM With Back Gate to Increase Static Noise Margin and Its Application to Braille Sheet Display

An integrated system of organic FETs (OFETs) and plastic actuators is proposed, and it is applied to a Braille sheet display. Some circuit technologies are presented to enhance the speed and the lifetime for the Braille sheet display. An OFET SRAM is developed to hide the slow transition of the actuators. Developed five-transistor SRAM cell reduces the number of the bit lines by one-half and reduces the SRAM cell area by 20%. Pipelining the write-operation reduced the SRAM write-time by 69%. Threshold voltage control technology using a back gate increased the static noise margin of SRAM and compensated for the chemical degradation of the OFETs after 15 days. The oscillation frequency tuning range from -82% to +13% in a five-stage ring oscillator is also demonstrated with the threshold voltage control technology. The overdrive techniques for the driver OFETs reduced the transition time of the actuator from 34 s to 2 s. These developed circuit technologies achieved the practical 1.75-s operation to change all 144 Braille dots on Braille sheet display and will be essential for the future large area electronics made with OFETs