Hitoshi Umezawa

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.

Nanofabrication on Hydrogen-Terminated Diamond Surfaces by Atomic Force Microscope Probe-Induced Oxidation.

Field-assisted local oxidation on a hydrogen-terminated (001) diamond surface was performed using an atomic force microscope (AFM). Anodic oxidation by a surface water meniscus layer is suggested to account for this oxidation process. Through the oxygenated area, Fowler-Nordheim (F-N) tunneling current was observed. The difference in electron affinity between the hydrogen-terminated surface and the oxygenated area was confirmed by scanning electron microscopic (SEM) observations.

Over 20-GHz cutoff frequency submicrometer-gate diamond MISFETs

Submicrometer-gate (0.2-0.5-/spl mu/m) diamond metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated on an H-terminated diamond surface. The maximum transconductance in dc mode reaches 165 mS/mm, while the average transconductance is 70 mS/mm in submicrometer-gate diamond MISFETs. The highest cutoff frequency of 23 GHz and the maximum frequency of oscillation of 25 GHz are realized in the 0.2-/spl mu/m-gate diamond MISFET. From the intrinsic transconductances or the cutoff frequencies, the saturation velocities are estimated to be 4/spl times/10/sup 6/ cm/s in the submicrometer-gate FETs. They are reduced by gate-drain capacitance and source resistance.

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.

Surface-modified Diamond Field-effect Transistors for Enzyme-immobilized Biosensors

The enzyme sensors using electrolyte-solution-gate diamond field effect transistors (SGFETs) have been developed for the first time. The hydrogen-terminated surface channel of the FETs was modified into partially aminated and oxygen-terminated (H-A-O-terminated) with irradiation of ultraviolet in an ammonia environment. The pH response of that is obtained about 50 mV/pH at pH 2–10. The concentration of substrates (urea or glucose) in the electrolyte solution has been detected by the pH change due to the bio-catalyzed effect of enzyme (urease or glucose oxidase), which is immobilized on the channel of SGFETs. The sensitivity of urea and glucose is approximately 30 mV/decade and 20 mV/decade respectively.

Potential applications of surface channel diamond field-effect transistors

Abstract In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm −1 . High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f T and maximum frequency of oscillation f max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f T and 30 GHz of f max .

Control of adsorbates and conduction on CVD-grown diamond surface, using scanning probe microscope

Nanofabrication on a hydrogen-terminated diamond surface is performed by controlling surface adsorbates, using a scanning probe microscope (SPM) technique. Insulated areas are successfully obtained by changing hydrogen termination to oxygen termination. The Auger electron spectrum (AES) indicated the presence of oxygen adsorbed on the modified surface area. A small isolated conductive area is fabricated, and using this structure, a metal–insulator–metal (MIM) diode-like I–V characteristic is observed. Anodic oxidation using the surface water is also suggested for our experimental results.

High-Performance Diamond Metal-Semiconductor Field-Effect Transistor with 1 µm Gate Length

High-performance metal-semiconductor field-effect transistors (MESFETs) using the p-type surface conductive layer on homoepitaxial diamond are demonstrated. The maximum transconductance is 110 mS/mm, which is the highest value ever reported in diamond FETs. This value exceeds the normal transconductance of a Si–metal-oxide semiconductor field-effect transistors (MOSFET) with equivalent gate length. The transconductance of the present diamond FETs is proportional to the reciprocal of gate length. Accordingly, the characteristics can be improved by the refinement of gate length. By using an appropriate FET fabrication process, it is expected that the transconductance of a diamond MESFET exceeds 500 mS/mm at gate lengths less than 0.2 µm.

Cu/CaF2/Diamond Metal-Insulator-Semiconductor Field-Effect Transistor Utilizing Self-Aligned Gate Fabrication Process

High-performance metal-insulator-semiconductor field-effect transistors (MISFET) on hydrogen-terminated homoepitaxial diamond films are demonstrated. The gate insulator is evaporated CaF2 which does not cause interface states. This is the first study of a CaF2/diamond MISFET fabricated by a self-aligned gate fabrication process by which the gate length and the source gate spacing are effectively reduced. The maximum transconductance is 86 mS/mm, which is the highest value in diamond MISFETs at present.

High performance diamond MISFETs using CaF2 gate insulator

Abstract A cut-off frequency of 15 GHz and a maximum frequency of oscillation of 20 GHz are realized in a 0.4-μm gate diamond metal–insulator–semiconductor field-effect transistor (MISFET). The cut-off frequency is the highest value for diamond FETs ever reported. The RF characteristics of the MISFETs are higher than those of metal–semiconductor FETs at the same gate lengths. The CaF2 gate insulator improves the carrier mobility according to the Hall measurement system. The mobility increases in the surface conductive layer result in high RF performance. The source–gate passivation of CaF2 results in the high DC transconductance because of the reduction of series resistances. A cut-off frequency of more than 30 GHz is expected with the gate minimization and the CaF2 passivation of source–gate and gate–drain spacings.