Papers by Author: Hitoshi Umezawa

Abstract: In this study, we conducted in-situ measurements on a SiC JFET operational amplifier operating under gamma-ray irradiation. It shows that the radiation did not affect the output waveform or voltage gain, but shifted the output offset voltage. This shift may result mainly from holes generated by irradiation and trapped in the oxide layer, which modified the I-V characteristics of the level-shifting diodes. It can be compensated by applying bias voltage, and it may also be prevented by optimizing the diode structure and/or circuit topology.

Abstract: Currently, silicon carbide (SiC) is widely recognized as a wide bandgap semiconductor, with expanding applications in harsh environments, such as high temperature and radiation exposure. In this study, we fabricated a planar structure 4H-SiC gate-all-around junction field-effect transistor (JFET), wherein the channel region is formed through ion implantation at varying doses. We successfully produced both normally-on and normally-off JFETs. Moreover, we constructed a JFET commonsource amplifier. The amplifiers achieved a maximum gain of -226.7 (47.1 dB) at a supply voltage of V_DD = 30 V.

Abstract: A monocrystalline diamond substrate was bonded with a Si substrate employing a direct bonding technique. The diamond and Si surfaces were functionalized with hydroxyl (–OH) groups and subsequently bonded by the thermal dehydration reaction across the bonding interface. When a diamond (111) surface was treated with a mixture of H₂SO₄ and H₂O₂, it generated an atomic bond of C–O–Si with an oxygen-plasma-irradiated Si substrate. The bonding technique of diamond using the H₂SO₄/H₂O₂ mixture is expected to contribute to the future integration of diamond and semiconductor substrates because it allows low-temperature bonding in atmospheric air with negligible crystallinity damage.

Abstract: Lateral gate depletion expansion towards drain contact has been analyzed on p-type diamond metal-semiconductor field effect transistor by electron beam induced current. The investigation was restricted to a closed channel to simplify the study and to directly observe the expansion of the lateral depletion region. The experimental data agreed with the theoretical model given in the literature.

Abstract: A field-plate structure is applied to vertical diamond Schottky barrier diode. A sputtered Al₂O₃ with 0.2 µm thickness is utilized for field-plate insulator. Fabricated diamond VSBD shows low leakage characteristics. Accordingly, the breakdown voltage is improved from 900V to 1,800V.

Abstract: Wide band gap semiconductors have been attracted as the material for fabricating power switching devices to obtain lower power conversion loss in high voltage circuit, and to operate harsh environment of high temperature. This paper focuses on diamond as the wide band gap semiconductor material and elucidates the dynamic characteristics in switching operation. To this end, Schottky barrier diode (SBD) is fabricated with p type diamond semiconductor and static I-V characteristics is evaluated. Then, the switching operation of diamond SBD is demonstrated, and forward current dependency of the recovery phenomena is characterized. The diamond SBDs show superior fast switching capability with low reverse recovery current, which is inherent in uni-polar switching device.

Abstract: Because of the superior material properties of diamond, high performance in high-temperature power device application is demonstrated in both the computational and the experimental studies. A calculated Baliga limit of diamond Schottky barrier diode (SBD) based on 1D model indicates that an increase in on-resistance is highly expected within the high blocking voltage region and will be remarkable at high temperature conditions. Because of the high barrier height of diamond SBDs, the reverse leakage current is suppressed even at high temperatures. From the high-temperature stability test, stable interfaces of metal/diamond contact with constant Schottky barrier height have been confirmed. The SBD works for more than 1500 hrs at 400oC.

Abstract: Diamond is a hopeful candidate for power switching device which can operate at high temperature as a “Cooling system free” device, yet at a high current. Recently we have developed a 3D diamond CVD growth method coupled with a sophisticated “direct wafer fabrication technique” to fabricate diamond wafer without slicing. Currently, half inch size single crystal diamond substrates are available for R&D of diamond device. Using this technique, we have increased the device fabrication size from 3x3mm2 to half inch wafer. In this paper, we present the results of measurements on the first device fabricated on a half inch size CVD substrate. We have carried out the first device characteristics mapping for diamond, and have observed the influence of substrate characteristics on the SBD characteristics.

Abstract: Device size scaling of pseudo-vertical diamond Schottky barrier diodes (SBDs) has been characterized for high-power device applications based on the control of doping concentration and thickness of the p- CVD diamond layer. Decreasing parasitic resistance on the p+ layer utilizing lithography and etching makes possible to get a constant specific on-resistance of less than 20 mOhm-cm2 with increasing device size up to 200 µm. However, the leakage current under low reverse bias conditions is increased markedly. Due to the increase in the leakage current, the reverse operation limit is decreased from 2.4 to 1.3 MV/cm when the device size is increased from 30 to 150 µm. If defects induce an increase in leakage current under the reverse conditions, the density of the defects can be estimated to be 104–105/cm2. This value is 5–10 times larger than the density of dislocations in single crystal diamond substrate.

Abstract: Diamond is nominated as a material candidate for future high power device due to its superior material properties and resulting very high FOM. In this paper, our recent progresses and the expected possibilities of diamond for power electronics applications are introduced as short review. Firstly for the epitaxial growth, by adopting step-flow epitaxial growth by off- angle substrate with optimized growth conditions, we have succeeded in reducing these killer defects almost six orders from 106cm-2 to almost 100cm-2 levels. For the substrate, our recently developed technology to fabricate diamond plates from bulk, 12x13mm2 size are available to use, that can avoid fabrication difficulties with small size substrate. Secondly for the device, primitive studies showed possibly for the advantage of diamond such as low reverse leakage current, high temperature and high current density operation.