Fabrication and Performance of 16-kV Ultrahigh-Voltage SiC Power Devices
We have been working on a SiC p-channel IGBT with a BV of 10 kV  as well as a PiN diode with a BV of 13 kV . For these devices, a high-quality n++ substrate could be used for device fabrication. However, the crystal quality of the p++ SiC substrate for the purpose of fabricating an n-channel IGBT is currently very poor with a high micropipe density and high resistivity using it as a collector. Moreover, the channel mobility for a SiC-MOSFET is still very low compared with that of a Si-MOSFET because of its 10-times higher interface-state density (Dit).
To solve these problems related to n-channel SiC-IGBTs, we employed a heavily doped epitaxial p++ layer as a substrate and an implantation and epitaxial MOSFET (IEMOSFET) [5, 6] as a MOSFET structure, which is called a flip-type IE-IGBT. For the substrate, we attempted to fabricate a flip-type wafer utilizing a p++ epitaxial layer as a substrate . First, a 150-mm-thick n–-type drift layer was grown on the Si-face n++ substrate after a buffer layer was formed. After the p+ collector layer was grown, the p++ substrate layer was grown to a thickness greater than 200 µm. Then, we removed the n++ substrate, turned the substrate over, and polished the surface using the CMP process. To overcome the low channel mobility of the SiC-MOSFETs, we proposed the IEMOSFET utilizing the 4H-SiC (000-1) carbon face, which has a high channel mobility greater than 100 cm2/(Vs) . The bottom and top of the p-well of the IE-MOSFET are formed by ion implantation and epitaxial growth, respectively. The smooth surface of the top of the p-well enables high channel mobility. A TCAD simulation was employed to optimize the design of the active area and edge termination to obtain a low forward-voltage drop (Vf) and an ultrahigh breakdown voltage.
As a result, we successfully fabricated an IE-IGBT with a low Vf of 5.0 V at 100 A/cm2 with a BV greater than 16 kV . At the same time, we achieved good threshold voltage (Vth) stability and a low current-density dependence on the temperature. An ultrahigh-voltage power module was assembled to evaluate the dynamic behavior of the IE-IGBT and consisted of a tungsten base plate, a DBC base with Si3N4on it, and a copper electrode. The dynamic switching performance of the combination of the ultrahigh-voltage IE-IGBT and PiN diode will be presented.
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