PCIM Europe 2012 Nuremberg: PCIM is internationally renowned for the technical excellence of its conference program and the papers that form it. Therefore, the event’s Best Paper Award is no minor accolade.
Congratulations this year go to Keiji Okumura of the ROHM Company, Japan, for his paper, “Ultra low Ron SiC Trench devices.” Selected from a total of 230 contributions, it focuses on next-generation silicon-carbide (SiC) planar MOSFETs, trench structure Schottky diodes, and trench MOSFETs.
Some companies have already begun mass production of SiC planar MOSFETs for the sake of lower switching losses in high-voltage applications such as converters and inverters.
But as the paper points out, on resistance increases when current flows into the parasitic body diodes of these mass produced MOSFETs. This is because the parasitic PN body diodes, with the base plane dislocation, induce expansion of stacking faults in 4H-SiC epilayers and degrade the on resistance of both the body diodes and MOSFETs.
This process creates an obstacle for application in circuits that require current penetration from source to drain such as converters and inverters at the mass production level.
However, ROHM’s developed SiC planar MOSFETs have suppressed the degradation of parasitic PN junction diodes even if forward current penetrates into the PN junction diodes. Also, SiC Schottky diodes are attractive devices for reducing switching losses in high-voltage applications. The reduction of conductive losses is also required to improve efficiency.
But as the paper identifies, SiC Schottky diodes have higher forward voltage drop than silicon PN junction diodes because SiC Schottky diodes need high barrier heights to block leakage current since SiC has a breakdown strength 10 times greater than silicon.
According to Okumura, SiC Schottky diodes with newly developed trench structures successfully showed lower forward voltage than conventional SiC diodes while keeping leakage current at an acceptable level.
Finally, SiC trench MOSFETs can have lower conductive losses than planar MOSFETs because planar MOSFETs have JFET regions that increase the on resistance. However, the trench MOSFETs had issues regarding oxide breakdown at the trench bottom during high drain-source voltage application.
Developed SiC MOSFETs with a double-trench structure have improved device reliability while maintaining ultra-low on resistance since the new structure effectively reduced the highest electric field at the bottom of the gate trench, preventing gate oxide breakdown.