Papers by Author: Katsunori Asano

Abstract: Thick multi-layer 4H-SiC epitaxial growth was investigated for very high-voltage Si-face p-channel insulated gate bipolar transistors (p-IGBTs). The multi-layer included n⁺ buffer, p⁺ field stop, and thick p^- drift layers. Two processes were employed to enhance the carrier lifetime of the p^- drift layer: carbon ion implantation/annealing and hydrogen annealing, and the enhanced carrier lifetime was confirmed by the open-circuit voltage decay measurement. Using the grown thick multi-layer 4H-SiC, simple pin diodes were fabricated instead of p-IGBTs to demonstrate efficient conductivity modulation in the thick p^- drift layer. While the on-state voltage was high at room temperature, it decreased significantly at elevated temperatures, and attained 3.5 V at 100 A/cm² at 200°C for the diode with the carrier lifetime enhancement processes, indicating sufficient conductivity modulation.

Abstract: We successfully fabricated 13-kV, 20-A, 8 mm × 8 mm, drift-free 4H-SiC PiN diodes. The fabricated diodes exhibited breakdown voltages that exceeded 13 kV, a forward voltage drop of 4.9–5.3 V, and an on-resistance (R_onA_active) of 12 mW·cm². The blocking yield at 10 kV on a 3-in wafer exceeded 90%. We investigated failed devices using Candela defect maps and light-emission images and found that a few devices failed because of large defects on the chip. We also demonstrated that the fabricated diodes can be used in conducting high-voltage and high-current switching tests.

Abstract: The epitaxial growth of thick multi-layer 4H-SiC to fabricate very high-voltage C-face n-channel IGBTs is demonstrated using 3-inch diameter wafers. We employ an inverted-growth process, which enables the on-state voltage of resultant IGBTs to be reduced. Furthermore a long minority carrier lifetime (> 10 μs) and a low-resistance p⁺ epilayer can reduce the forward voltage drop of the IGBTs. The small forward voltage drop is demonstrated particularly at high temperatures by fabricating and characterizing simple pin diodes using the epi-wafer.

Abstract: The reverse recovery characteristics of a 4H-SiC PiN diode under higher voltage and faster switching are investigated. In a high-voltage 4H-SiC PiN diode, owing to an increased thickness, the drift region does not become fully depleted at a relatively low voltage Furthermore, an electron–hole recombination must be taken into account when the carrier lifetime is equal to or shorter than the reverse recovery time. High voltage and fast switching are therefore needed for accurate analysis of the reverse recovery characteristics. The current reduction rate increases up to 2 kA/μs because of low stray inductance. The maximum reverse voltage during the reverse recovery time reaches 8 kV, at which point the drift layer is fully depleted. The carrier lifetime at the high level injection is 0.086 μs at room temperature and reaches 0.53 μs at 250 °C.

Abstract: The reliability of three kinds of high heat-resistant resins has been evaluated under high temperatures. These resins were applied to insulation substrates and a high temperature storage test has been carried out. The insulation performance of the resins was evaluated by applying 20 kV between a pair of electrodes on the substrate covered with resin. The insulation performance at 20 kV was maintained in samples with two of the three kinds of resins for 1,000 hours at 225oC. In a higher temperature storage test at 250oC, samples with one of the kinds of resin were not able to maintain insulation of 20 kV for 200 hours, while the two remaining resins were not able to maintain the insulation for 1,000 hours. In most samples that were not able to maintain the insulation, cracks or detachments were seen. Hardening caused by oxidation of the resin and differences in the coefficient of linear thermal expansion (CTE) are considered as causes of the cracks or detachments. It is thought to be necessary to lower the CTE of the resin and inhibit its oxidation in order to use it at more than 250oC for long periods of time.

Abstract: Insulating properties of package for ultrahigh-voltage, high-temperature devices have been investigated. While all the packages have enough insulating strength at room temperature, deterioration of the insulating property at high temperature has been found with some packages. The authors have found that this deterioration is attributed to degrade the insulation property of AlN ceramics for DBC substrate at high temperature and that there is a various degree in the deterioration.

Abstract: Temperature dependence simulations of forward characteristics for 4H-SiC pin diodes with Shockley-type stacking faults are performed in order to investigate the mechanism of the TEDREC phenomena. The forward voltage drops of both n-type and p-type drift layers at room temperature increase as the length of the Shockley-type stacking fault increases. When the diodes are compared to each other at the same temperature, the differences between the forward voltage drops do not change significantly up to 150 oC, but the differences suddenly narrow in the range from 150 °C to 200 °C. The Shockley-type stacking fault prevents current from flowing at room temperature. The current, however, flows throughout the drifted diode when the temperature is raised.

Abstract: The electrical characteristics of 4H-SiC pin diodes with 8H-type in-grown stacking faults are investigated. The pin diodes have epilayers with low Z_1/2 center concentration formed by using the carbon implantation process. The forward voltage drops of the diode with 8H-type in-grown stacking faults are larger than those of the diode without a 8H-type in-grown stacking fault. At room temperature, the differential on-resistance of the pin diode with 8H-type in-grown stacking faults is larger than the value calculated from donor concentration in the drift layer by using the current transportation model of the unipolar device. Meanwhile, the differential on-resistances of the pin diode with 8H-type in-grown stacking faults decrease with an increase in temperature and become smaller than the calculated value at temperature of more than 200 °C.

Abstract: The transient electrical characteristics of the forward recovery and reverse recovery characteristics of lifetime-controlled high blocking voltage 4H-SiC pin diodes by electron irradiation are investigated. Even at a heavy electron dose of 1×1014 cm-2, the forward voltage overshoot of a 4H-SiC pin diode is lower than that of a 2 kV/100 A class Si fast diode. As for the reverse recovery characteristics, small reverse recovery current and fast reverse recovery time are obtained by electron irradiation. The reduction ratio of recovery loss can therefore exceed the increase ratio of steady-state loss by electron irradiation.

Abstract: The forward voltage drops of pin diodes with the carbon implantation process or thermal oxidation process using a drift layer of 120 μm thick are around 4.0 V and are lower than those with the standard process. The reverse recovery characteristics of diodes with the standard process or carbon implantation at room temperature show almost the same tendency. In the reverse recovery characteristics at 250 oC, pin diodes with carbon implantation process, however, have the longer reverse recovery time than those with the standard process. These characteristics indicate that a recombination path other than the bulk carrier lifetime, such as the interfaces or the surface recombination, becomes dominant in the reverse recovery characteristics at room temperature.