Papers by Author: Takahiro Makino

Abstract: Characteristics of 4H-SiC nMOSFETs with arsenic-doped S/D and NbNi silicide contacts in harsh environments of high-temperature up to 450°C, and high gamma-ray radiation up to over 100 Mrad, were investigated. At high temperature, field effect mobility increased as proportional to T^3/2, and threshold voltage was shifted with temperature coefficients of -4.3 mV/K and -2.6 mV/K for oxide thicknesses of 10 nm and 20 nm, respectively. After Co60 gamma-ray exposure of 113 Mrad, the field effect mobility was varied within 8% for oxide thickness of 10 nm, however for 20 nm oxide thickness, this variation was 26%. The threshold voltage shifts were within 6%.

Abstract: Gamma-ray irradiation effects of motor-driver circuit composed of SiC MOSFETs under motor driving with different PWM frequencies were investigated. The driving current and voltage waveforms were normal when the irradiation exceeded 1.1 MGy at PWM frequency of 10 kHz. In addition, the motor was still rotating in this total dose. We compared the irradiation responses of SiC MOSFETs between the cases of driving states and no bias. The drain current – gate voltage characteristics with no bias shifted to the negative voltage side wider than the driving states. Also the leakage current in the case of driving state is fewer than that of no bias.

Abstract: The charge enhancement in SiC-Schottky Barrier Didoes (SBDs) with different epi-layer thicknesses under the condition of the single-species ion irradiation was simulated to find out the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. The value of ion induced charge depended on the thickness of epitaxial-layer in the SBDs. The simulation result suggests that the impact ionization is one of the key effects to lead ion induced charge enhancement.

Abstract: Radiation response of 4H-SiC vertical power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) was investigated at 150°C up to 10.4 MGy. Until irradiation at 1.2 MGy, the drain current – gate voltage curves of the SiC MOSFETs shifted to the negative voltage side, and the leakage of drain current at gate voltages below threshold voltage increased with increasing absorbed dose. However, no significant change in the electrical characteristics of SiC MOSFETs was observed at doses above 1.2 MGy. For blocking characteristics, there were no degradations of the SiC MOSFETs irradiated at 150°C even after irradiated at 10.4 MGy.

Abstract: We present EDMR (electrically detected magnetic resonance) observations on “C-face defects” in C-face 4H-SiC MOSFETs. We found that negative threshold-voltage shifts of C-face MOSFETs are increased in association with EDMR signals of C-face defects as well as with the dissociation of hydrogen atoms induced by gamma-ray irradiation.

Abstract: The charge induced in SiC-SBDs with different epi-layer thicknesses by ion incidence was measured to understand the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. SiC SBD of which epitaxial-layer thicknesses is close to ion range show larger anomalous charge collection than SBD with thicker epi-layer although the former one has lower electric field than the later one. The gains of collected charge from the SBDs suggest that the impact ionization under 0.16 - 0.18 MV/cm of the static electric field in depletion layer is not dominant mechanisms for the anomalous charge collection. It is suggested that the epitaxial-layer thickness and ion-induced transient high electric field are key to understand the anomalous charge collection mechanisms in SBDs.

Abstract: Effects of gamma-ray irradiation and subsequent thermal annealing on the characteristics of vertical structure power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) fabricated on 4H-SiC were studied. After irradiation at 1.2 MGy, the drain current – gate voltage curves of the MOSFETs shifted to the negative voltage side and the leakage drain current at inverse voltage increased. No significant change in the degraded electrical characteristics of SiC MOSFETs was observed by room temperature annealing. The degraded characteristics of SiC MOSFETs began to recover by annealing above 120 °C, and their characteristics reached almost the initial ones by annealing at 360 °C.

Abstract: The critical electric field (E_cr) of the gate oxide in 4H-Silicon Carbide (SiC) MOSFETs was measured under inversion bias conditions with ion irradiation. The Linear Energy Transfer (LET) dependence of the E_cr at which the gate oxide breakdown occurred in these MOSFETs was evaluated. The linear relationship between the E_cr^-1 and LET was observed for SiC MOSFETs. The slope of the LET-1/E_cr for SiC MOSFETs is almost the same that of the LET-1/E_cr lines for SiC MOS capacitors. The V_ds dependence of E_cr was also evaluated. The correlation between the direction of electric field of drain-source region and direction of ion incidence affects to instability of E_cr.

Abstract: The leakage currents through the gate oxide of MOS capacitors fabricated on n-type 4H-Silicon Carbide (SiC) was measured under accumulation bias conditions with heavy-ion irradiation. The Linear Energy Transfer (LET) dependence of the critical electric field (E_cr) at which dielectric breakdown occurred in these capacitors with two different oxide thicknesses was evaluated. The MOS capacitors with thin gate oxide showed higher E_cr values than those with thick gate oxide. The linear relationship between the reciprocal E_cr and LET was observed for both MOS capacitors. The slope of LET dependence of 1/E_cr for SiC MOS capacitors was smaller than that for Si, suggesting that SiC MOS devices are less susceptible to single-event gate rupture (SEGR) than Si MOS devices.

Abstract: In order to test the response of radiation-induced current with wide range of dose rate, a Silicon Carbide (SiC) dosimeter is exposed to gamma-rays emitted from a ⁶⁰Co source. The SiC dosimeter in this study is made of a high purity semi-insulating 4H-SiC with nickel and aluminum electrodes. We have successfully demonstrated that the radiation-induced currents in the dosimeter show a linear relationship with the dose rate, and are repeatable and stable.