Hiroyuki Fujisawa

Development of Ultrahigh-Voltage SiC Devices

Ultrahigh-voltage silicon carbide (SiC) devices [p-i-n diodes and insulated-gate bipolar transistors (IGBTs)] and switching test have been investigated. As a result, we have succeeded in developing a 13-kV p-i-n diode, 15-kV p-channel IGBT, and 16-kV flip-type n-channel implantation and epitaxial IGBT with a low differential specific on-resistance (Rdiff,on). It was revealed that a power module fabricated using a nanotech resin, Si3N4 ceramic substrate, and W base plate was suitable for ultrahigh voltage and high temperature. A switching test was carried out using a clamped inductive load circuit, which indicated that the energy loss of a circuit with ultrahigh-voltage SiC devices is lower than that of Si devices.

Fabrication of a P-Channel SiC-IGBT with High Channel Mobility

We fabricated and characterized an ultrahigh voltage (>10kV) p-channel silicon carbide insulated gate bipolar transistor (SiC-IGBT) with high channel mobility. Higher field-effect channel mobility of 13.5 cm2/Vs was achieved by the combination of adopting an n-type base layer with a retrograde doping profile and additional wet re-oxidation annealing (wet-ROA) at 1100°C in the gate oxidation process. The on-state characteristics of the p-channel SiC-IGBT at 200°C showed the low differential specific on-resistance of 24 mΩcm2 at VG = -20 V. The forward blocking voltage of the p-channel SiC-IGBT at 25°C was 10.2 kV a the leakage current density of 1.0 μA/cm2.

Analysis of Structural Defects in the 4H-SiC Epilayers and their Influence on the Electrical Properties

We fabricated epi pn diodes and investigated the correlation between defects included in the 4H-SiC epilayers and the leakage current of the pn diodes. Threading edge dislocations, screw dislocations, basal plane dislocations and in-grown stacking faults existed in the epilayers. From EL images of the diodes under reverse bias voltage, it was found that a line of edge dislocations forming a grain boundary and some screw dislocations caused an increase in leakage current. We also mention the influence of dissociated micropipes.

Low V f and highly reliable 16 kV ultrahigh voltage SiC flip-type n-channel implantation and epitaxial IGBT

Flip-type n-channel implantation and epitaxial (IE)-IGBT on 4H-SiC carbon face with an epitaxial p++ collector layer was investigated. In this study, we employed the IEMOSFET as a MOSFET structure with original wet gate oxidation method, to realize high channel mobility. We were able to achieve an ultrahigh blocking voltage of more than 16 kV, extremely low forward voltage drop of 5 V at 100 A/cm2 and small threshold voltage shift (<; 0.1 V). These characteristics are useful for Smart Grid and HVDC systems, the use of which would realize a low carbon emission society.

Effect of Current-Spreading Layer Formed by Ion Implantation on the Electrical Properties of High-Voltage 4H-SiC p-Channel IGBTs

High-voltage SiC p-channel insulated-gate bipolar transistors (p-IGBT) utilizing current-spreading layer (CSL) formed by ion implantation are fabricated and their properties characterized. A high blocking voltage of 15 kV is achieved at room temperature by optimizing the JFET length. An ampere-class p-IGBT exhibited a low forward voltage drop of 8.5 V at 100 A/cm2 and a low differential specific on-resistance of 33 mΩ cm2 at 250 °C, while these values were high at room temperature. For further reduction of the forward voltage drop in the on-state and temperature stability, the temperature dependence of the JFET effect and carrier lifetime in p-IGBTs are investigated. Optimization of the JFET length using an epitaxial CSL, instead of applying ion implantation and lifetime enhancement, could lead to a further reduction of the forward voltage drop.

Device Performance and Switching Characteristics of 16 kV Ultrahigh-Voltage SiC Flip-Type n-Channel IE-IGBTs

Ultrahigh-voltage SiC flip-type n-channel implantation and epitaxial (IE)-IGBTs were developed, and the static and dynamic performance was investigated. A large device (8 mm × 8mm) with a blocking voltage greater than 16 kV was achieved, and an on-state current of 20 A was obtained at the low on-state voltage (Von) of 4.8 V. RonAdiff was 23 mΩ·cm2 at Von = 4.8 V. In order to evaluate the switching characteristics of the IE-IGBT, ultrahigh-voltage power modules were assembled. A chopper circuit configuration was used to evaluate the switching characteristics of the IE-IGBT. Smooth turn-off waveforms were successfully obtained at VCE = 6.5 kV and ICE = 60 A in the temperature range from room temperature to 250°C.

13-kV, 20-A 4H-SiC PiN Diodes for Power System Applications

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 (RonAactive) 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.

Origin analysis of expanded stacking faults by applying forward current to 4H-SiC p?i?n diodes

Stacking faults expanded by the application of forward current to 4H-SiC p–i–n diodes were observed using a transmission electron microscope to investigate the expansion origin. It was experimentally confirmed that long-zonal-shaped stacking faults expanded from basal-plane dislocations converted into threading edge dislocations. In addition, stacking fault expansion clearly penetrated into the substrate to a greater depth than the dislocation conversion point. This downward expansion of stacking faults strongly depends on the degree of high-density minority carrier injection.

Dynamic characteristics of large current capacity module using 16-kV ultrahigh voltage SiC flip-type n-channel IE-IGBT

4H-SiC carbon face flip-type n-channel implantation and epitaxial (IE)-IGBT with an epitaxial p++ substrate was developed and its switching test was carried out. We were able to achieve an ultrahigh blocking voltage greater than 16 kV, extremely low Von (6.35 V at 20 A), and good temperature stability. The switching operation was achieved by connecting three IGBTs in parallel, with a total ICE of 60 A and VCE 5 kV. The turn-off loss and turn-on loss were about 220 mJ and 120 mJ, respectively at room temperature. They show low switching loss of ultrahigh voltage SiC IE-IGBT and the possibility of large scale module with parallel connection.

High Voltage and Fast Switching Reverse Recovery Characteristics of 4H-SiC PiN Diode

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.