Invited paperReliability of Diode-Integrated SiC Power MOSFET(DioMOS)
Introduction
Silicon Carbide (SiC) is a wide bandgap semiconductor that is very suitable for power switching applications especially at high voltage and current. Technical breakthroughs on the crystal growth and the consequent development of the processing technologies have resulted in successful commercialization of SiC Schottky barrier diodes. SiC MOS transistors with superior performances to existing Si power transistors have also been demonstrated and thus wide spread use of SiC transistors in the near future would be expected with the establishment of the reliability. Although the performances in DC and/or switching of the reported SiC MOSFETs are sufficiently good, there remains an issue of high fabrication cost because of the inherently high cost of the SiC substrate. Thus establishment of good enough reliability with reduced fabrication cost is very critical for the widespread use of SiC MOS transistors. So far, diode-integrated MOSFET (DioMOS) has been proposed as a solution to reduce the area of SiC for practical switching by the integration of the reverse diode on to the transistor [1], [2], [3], [4]. Here additionally formed n-type epitaxial channel layer serves current path for the reverse conduction in the DioMOS so that an external diode to flow the reverse current can be eliminated. Elimination of external diode leads to a reduction of the total number of devices in the system resulting in significant cost down. The DioMOS with satisfactorily good reliability would help the wide spread use of SiC power switching transistors.
In this paper, status of the reliability of the SiC DioMOS is summarized after reviewing the reliability issues of conventional SiC MOS transistors. With the help of channel diode avoiding current flow to the body diode that causes the degradation in conventional MOSFET, the DioMOS exhibits very stable characteristics at high temperature and under high drain voltage of 1200 V. This indicates that the DioMOS will greatly help the widespread use of SiC transistor taking advantage of the reduction of the total area of SiC contributing to reduce the total system cost.
Section snippets
Reliability issues of conventional SiC-MOSFET
SiC power transistors that have been widely investigated so far are vertical MOS (Metal-Oxide-Semiconductor) transistors. The structure as shown in Fig. 1 is identical with that of conventional Si power MOS transistors called DMOS (Double diffusion MOS). Issues of reliability in conventional SiC DMOS are also described in Fig. 1, of which the details are described below.
The most critical reliability issue of SiC MOFET so far has been the stability of MOS gate. Typically, the threshold voltages
Structure and performances of diode-integrated SiC-MOSFET (DioMOS)
Here, a new structure of a SiC MOSFET called as DioMOS is described. The DioMOS has an extra channel layer underneath the gate oxide that serves a current path for the reverse conduction. This prevents the current from flowing onto the pn-junction-based and thus solves the reliability issue. The DioMOS can also serve the low built-in voltage diode for the reverse conduction by optimization of the channel design. Thus the DioMOS can eliminate the conventional SiC SBD to flow the reverse current
Reliability of SiC DioMOS
Since the DioMOS is free from the degradation of body diodes owing to the above-mentioned channel design, the shift of the threshold voltages is the only remaining reliability issue. The efforts to improve the interfacial properties at SiO2/SiC successfully improve the gate reliability as described below. Optimization of the thermal oxidization and the subsequent NO annealing successfully suppresses the Vth shift together with high channel mobility of electrons at the oxide interface. The
Conclusions
Reliability of newly proposed SiC DioMOS (diode-integrated MOSFET) is presented after summarizing the reliability issues in conventional SiC MOSFETs. The advantages of DioMOS for practical application is the integration of the reverse diode by adding an epitaxial n-type channel layer can eliminates external SiC Schottky barrier diodes (SBDs) enabling the reduction of total area and cost of SiC and smaller size of the packaging. Improvements in the fabrication process of DioMOS mitigated the
Acknowledgments
The authors would like to acknowledge Mr. M. Hayashi, Mr. S. Karashima and Mr. R. Kunisato for the discussions and the help on the reliability tests of SiC DioMOS.
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