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Design and fabrication of high performance UMOSFETs in 4H SiC

Purdue University
Publication Date
  • Engineering
  • Electronics And Electrical
  • Design
  • Mathematics


Due to their high breakdown strength, power transistors made of wide bandgap semiconductors, for example diamond, III-V nitrides, and silicon carbide (SIC), can have 2–3 orders of magnitude lower specific on-resistance than silicon at the same blocking voltage. Therefore, these materials are attractive for high power and high blocking voltage applications. Among wideband semiconductors, SiC has attracted considerable attention because its process technology is relatively mature and because it is the only compound semiconductor with a high-quality native oxide, SiO2, making it possible to fabricate the whole family of MOS-gated devices in this material. In addition, higher thermal conductivity of SiC makes it a better choice than III-V Nitride for high power density applications. ^ However, due to the low inversion layer mobility in 4H-SIC, the on-resistance of SiC power MOSFETs reported to date has generally been dominated by the MOS channel resistance. In this thesis, we report studies that significantly improve the inversion layer mobility. In addition, we discuss the design challenges encountered when scaling down the channel length for UMOSFET, as well as the new layouts, isolation technology and geometry we developed for improving the performance of power transistors. We also demonstrate a manufacturable process and present measurements on short channel UMOSFETs made by this process. The electrical performances of these devices in the blocking states are close to the theoretical values, and this indicates our junction termination designs are successful. However, the on-state resistance is still high due to unexpected low inversion layer mobilities (in the 0.11cm2/Vs range). This low mobility can be partly explained by the combination of impurity scattering, Coulomb scattering, surface scattering, and mobile charge reduction in the inversion channel. Future mobility studies and structure changes are proposed to improve the on-state performance of short channel UMOSFETs. ^

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