Papers by Author: Jun Suda

Abstract: Effects of a parasitic region in SiC BJTs on conductivity modulation and a forced current gain (β_F) were investigated by using TCAD simulation with various device structures. By introducing an Al⁺-implanted region below the base parasitic region, β_F can be improved because the implanted region can reduce the base spreading resistance, leading to alleviation of debiasing effect. β_F in devices with various parasitic areas, whose base spreading resistances were reduced by the Al⁺-implantation, were compared. We found that β_F can be enhanced by expanding the parasitic area if the base spreading resistance is sufficiently reduced. The higher β_F is attributed to an expanded conductivity-modulated region. The collector current spreading in the collector layer and the hole injection from the parasitic region as well as from the intrinsic region can play a role to evoke the conductivity modulation. Thus, the larger parasitic region can expand the conductivity-modulated region, resulting in expansion of an active area and the enhancement of β_F.

Abstract: We studied the hydrogen passivation/depassivation of four types of intrinsic defects (EI5/6, HEI7/8, HEI9/10, and P6/7) in p-type and semi-insulating 4H-SiC by means of electron spin resonance (ESR) for examining the origin of career-lifetime-killing defects. We suggest that the HEI7/8 and P6/7 centers are the strongest candidate for the origin of the lifetime-killing defects.

Abstract: We fabricated electrostatically-excited single-crystalline 4H-SiC microcantilever resonators with various thicknesses and lengths. Their resonant characteristics were investigated from room temperature (RT) up to 600°C. The resonant frequency of the cantilevers decreased with increasing temperature. From the results, the temperature dependence of Young’s modulus of single-crystalline 4H-SiC was obtained, i.e., 3% decrement with increasing temperature from RT to 600°C. The cantilevers with different thicknesses showed different temperature dependences of the quality factor. A 2-μm-thick cantilever exhibited a high quality factor (Q) (250,000) at RT and the Q decreased to 6,000 at 600°C, which can be explained by thermoelastic damping. On the other hand, a Q of a 0.45-μm-thick cantilever was still high (50,000) even at 600°C.

Abstract: It was discovered that the oxidation rate for SiC depended on the conduction type. The oxidation was performed for SiC(0001) with nitrogen doping (n-type) in the range from 2×10¹⁶ cm^-3 to 1×10¹⁹ cm^-3, and aluminum doping (p-type) in the range from 2×10¹⁵ cm^-3 to 1×10¹⁹ cm^-3, exhibiting a clear dependence. For n-type SiC the oxide thickness increases for higher doping density, and for p-type the thickness decreases. Note that in the case of Si oxidation, there exists very little difference of oxidation rate between the conduction types in such low doping density, and the dependence is peculiar to SiC.

Abstract: We fabricated electrostatically actuated single-crystalline 4H-SiC microcantilever resonators. To realize a narrow gap between cantilevers and substrate, we etched a thin p-type SiC layer in n/p/n multilayer structure by doping-selective electrochemical etching. The resonant characteristics of the fabricated 4H-SiC microcantilevers were investigated under a vacuum condition. Electrostatic actuation of microcantilevers was successfully performed by applying 10 mV_rms ac voltage with 20 mV dc bias. The quality factor of 4H-SiC microcantilevers was above 100,000, which is about ten times higher than the quality factor of Si cantilevers with the same structure. Resonant characteristics were almost identical for mechanical actuation and electrostatic actuation.

Abstract: Characteristics of high-voltage lateral silicon carbide metal-oxide-semiconductor field-effect transistors (MOSFETs) with various reduced surface field (RESURF) structures were simulated. Breakdown voltage was enhanced from 5300 V for single-zone RESURF to 7400 V for two-zone, and to 7600 V for quasi-modulated RESURF MOSFETs.

Abstract: Ultrahigh-voltage SiC PiN diodes with an original junction termination extension (JTE) structure and improved forward characteristics are presented. A space-modulated JTE (SM-JTE) structure was designed by device simulation, and a high breakdown voltage of 26.9 kV was achieved by using a 270 μm-thick epilayer and 1050 μm-long JTE. In addition, lifetime enhancement process via thermal oxidation was performed to improve the forward characteristics. The on-resistance of the SiC PiN diodes was remarkably reduced by lifetime enhancement process. The temperature dependence of the on-resistance was also discussed.

Abstract: In freestanding n-type 4H-SiC epilayers irradiated with low-energy (250 keV) electrons at room temperature, the electron paramagnetic resonance (EPR) spectrum of the negative carbon vacancy at the hexagonal site, VC- (h), and a new signal were observed. From the similarity in defect formation and the spin-Hamiltonian parameters of the two defects, the new center is suggested to be the negative C vacancy at the quasi-cubic site, VC- (k). The identification is further supported by hyperfine calculations.

Abstract: Impact ionization coefficients of 4H-SiC were measured at room temperature and at elevated temperatures up to 200°C. Photomultiplication measurement was done in two complementary photodiodes to measure the multiplication factors of holes (M_p) and electrons (M_n), and ionization coefficients were extracted. Calculated breakdown voltage using the obtained ionization coefficients showed good agreement with the measured values in this study, and also in other reported PiN diodes and MOSFETs. In high-temperature measurement, breakdown voltage exhibited a positive temperature coefficient and multiplication factors showed a negative temperature coefficient. Therefore, extracted ionization coefficient has decreased which can be explained by the increase of phonon scattering. The calculated temperature dependence of breakdown voltage agreed well with the measured values not only for the diodes in this study, but also in PiN diode in other literature.

Abstract: We investigated the photoconductivity decay characteristics of p-type 4H-SiC bulk crystals grown by a modified Lely method by differential microwave photoconductance decay (μ-PCD) measurements using a 349-nm laser as an excitation source. We observed persistent photoconductivity (PPC) in the p-type SiC bulk crystals. The decay time at room temperature was 2600 μs. The decay time decreased with increasing temperature, resulting in 120 μs at 250oC, and the activation energy of the decay times was determined to be 140±10 meV. Long decay characteristics were also observed by below-band-gap excitation at 523 or 1047 nm. On the other hand, no PPC was observed in p-type homoepitaxial layers grown by hot-wall chemical vapor deposition.