Toshikatsu Sakai

Electrolyte-Solution-Gate FETs Using Diamond Surface for Biocompatible Ion Sensors

Diamond field effect transistors have operated in electrolyte solution for the first time. Since the hydrogen-terminated diamond surfaces are stable enough for the use as an electrochemical electrode, the diamond surface channels are exposed to the electrolyte in the transistor structure. A perfect pinch-off and saturated current-voltage characteristics have been obtained for bias voltages within the potential window. The threshold voltages are almost constant in electrolytes with different pH values of 7-13, indicating pH insensitiveness of the hydrogen-terminated diamond surface. Based on this pH insensitive surface, ion selective regions can be fabricated to form transistor-based biosensors.

Cl− sensitive biosensor used electrolyte-solution-gate diamond FETs

We have investigated the electrolyte-solution-gate field effect transisitors (SGFETs) used hydrogen terminated (H-terminated) or partially oxygen terminated (O-terminated) polycrystalline diamond surface in the Cl- and Br- ionic solutions. The H-terminated channel SGFETs are insensitive to pH values in electrolyte solutions. The threshold voltages of the diamond SGFETs shift according to the density of Cl- and Br- ions about 30 mV/decade. One of the attractive biomedical applications for the Cl- sensitive SGFETs is the detection of chloride density in blood or in sweat especially in the case of cystic fibrosis. The sensitivities of Cl- and Br- ions have been lost on the partially O-terminated diamond surface. These phenomena can be explained by the polarity of surface change on the H-terminated and the O-terminated surface.

Ozone-treated channel diamond field-effect transistors

Diamond field-effect transistors (FETs) whose channel is partially oxidized and highly resistive are fabricated by ozone treatment. These FETs are operated in electrolyte solutions. From XPS analyses, it is evident that hydrogen-terminated (H-terminated) diamond is partially oxygen-terminated (O-terminated) by ozone treatment. The quantification of surface oxygen in ozone-treated diamond is carried out. The quantification shows that the surface oxygen increases with an increase in ozone treatment time indicating the control of oxygen coverage. The partially O-terminated diamond surface channel is much less conductive compared with the H-terminated diamond. The ozone-treated FETs were operated stably even though the channel of the FETs becomes highly resistive. For the sensing of particular ions or molecules by the immobilization of sensing components, the control of surface termination is necessary.

Effect of iodide ions on the hydrogen-terminated and partially oxygen-terminated diamond surface

Abstract The effect of I − ions on the threshold voltages of the electrolyte-solution-gate diamond field-effect transistors (SGFETs) in KI solution is investigated. The threshold voltages of hydrogen-terminated (H-terminated) diamond SGFETs shift in the KI concentration range of 10 −6 –10 −1 M in aqueous solutions. The sensitivity of the H-terminated diamond surface to I − ions is higher than that to Cl − or Br − ions. However, the sensitivity to I − ions of the partially oxygen-terminated (O-terminated) diamond surface drastically decreases with ozone treatment. The mechanisms of these phenomena can be explained by the surface charge and the adsorbability of I − ions on the H-terminated and O-terminated diamond surfaces.

A 128×96 Pixel Stack-Type Color Image Sensor: Stack of Individual Blue-, Green-, and Red-Sensitive Organic Photoconductive Films Integrated with a ZnO Thin Film Transistor Readout Circuit

A color image was produced by a vertically stacked image sensor with blue (B)-, green (G)-, and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) array that uses a zinc oxide (ZnO) channel to read out the signal generated in each organic film. The number of the pixels of the fabricated image sensor is 128×96 for each color, and the pixel size is 100×100 µm2. The current on/off ratio of the ZnO TFT is over 106, and the B-, G-, and R-sensitive organic photoconductive films show excellent wavelength selectivity. The stacked image sensor can produce a color image at 10 frames per second with a resolution corresponding to the pixel number. This result clearly shows that color separation is achieved without using any conventional color separation optical system such as a color filter array or a prism.

Fabrication of Volcano-Structured Double-Gate Field Emitter Array by Etch-Back Technique

Volcano-structured double-gate field emitter arrays (VDG-FEAs) were fabricated using the etch-back technique. The fabrication process of the VDG-FEA is simple, and the height (hf) of the focusing electrode relative to the extraction gate electrode is easily adjusted by varying the etch-back time without high-resolution lithography. We have fabricated two types of VDG-FEAs with hf = +220 and 0 nm. The focusing characteristic is controlled by tuning hf. The decrease of the anode current is reduced for the VDG-FEA with lower hf under focusing condition producing the same beam spot size.

Effect of Cl- Ionic Solutions on Electrolyte-Solution-Gate Diamond Field-Effect Transistors

Diamond field-effect transistors (FETs) operate in electrolyte solutions having a wide pH range of 1–13. The FETs have been fabricated using a p-type surface conductive layer, where the diamond surface is exposed directly to the electrolyte solutions. From the drain current-gate voltage (Ids–Vgs) characteristics of the FETs, it appears that the threshold voltages of the FETs are independent of the pH value of the solution. In Cl- ionic solutions, however, the threshold voltages shift approximately 30 mV with a one-order-of-magnitude change of molar concentration of Cl- ions. This sensitivity of the FET to Cl- ions concrentration is observed in the 10-1–10-6 M range of potassium chloride (KCl) solutions.

A 128×96 Pixel, 50 µm Pixel Pitch Transparent Readout Circuit Using Amorphous In–Ga–Zn–O Thin-Film Transistor Array with Indium–Tin Oxide Electrodes for an Organic Image Sensor

Towards a signal readout circuit for a highly sensitive stack-type image sensor, an entire transparent thin-film transistor (TFT) array using an amorphous In–Ga–Zn–O channel and indium–tin oxide electrodes was fabricated. The pixel pitch and number of pixels were 50 µm and 128×96, respectively. The transmittance of the TFT array for visible light reached up to 85%. The array also showed good switching characteristics. A monochromatic image sensor with a zinc phthalocyanine organic photoconductive film was fabricated using this array, and it produced clear images at 30 frames per second with a resolution corresponding to the pixel number.

Continuous fabrication technology for improving resolution in RGB-stacked organic image sensor

With the goal of developing a compact, high-resolution color camera, we have been studying about a novel image sensor with three stacked organic photoconductive films: each film is sensitive to only one of the primary color components, and each has a signal readout circuit. In this type of image sensor, the acceptable focal depth is roughly estimated to be shorter than about 20 μm when the pixel pitch of the sensor is several μm. To reduce the total thickness of the stack-type sensor, a continuous fabrication technology that entails stacking continuously all layers from the bottom to the top of the sensor is necessary. In the continuously stacked sensor, the three organic layers separated by interlayer insulators are formed close to each other on a single glass substrate. In this paper, we describe the elemental technologies for the continuous fabrication of a stack-type organic image sensor consisting of improving the heat resistance of organic films and decreasing the fabrication temperature of the interlayer insulators and signal readout circuits. A 150°C heat-resistant organic photoconductive film can be obtained by using organic materials possessing high glass-transition temperatures, and low-temperature fabrication of the interlayer insulator can be accomplished by metal oxides using atomic layer deposition (ALD) at 150°C. The amorphous In-Ga-Zn-O thin-film transistors (TFT) are fabricated at a maximum temperature of 150°C by using Al2O3 gate insulator via ALD and a post-treatment. The resulting TFT has good transfer characteristics. A continuously-stacked organic image sensor can be fabricated by integrating these technologies.

Color image sensor using stacked organic photoconductive films with transparent readout circuits separated by thin interlayer insulator

We have been working on developing an image sensor with three stacked organic photoconductive films (OPFs) sensitive to only one primary color component (red—R, green—G, or blue—B); each OPF has a signal readout circuit. This type of stacked sensor is advantageous for the manufacture of compact color cameras with high-quality pictures, since color separation systems, such as prisms or color filter arrays, are eliminated because of the color selectivity of OPFs. To achieve a high-resolution stacked sensor, its total thickness should be reduced to less than 10 μm. In this study, we fabricated a color image sensor with R and G-sensitive OPFs by applying amorphous In-Ga-Zn-O thin-film transistor (TFT) readout circuits. A 10 μm-thick interlayer insulator separated the R and G-sensitive layers. The entire fabrication process for the device was implemented below 150°C to avoid damaging the OPFs. Output signals were successfully read from each OPF through the TFT circuit, and multi-color images were reproduced from the fabricated sensor.