This year Power Electronics Technology has selected a significant product development that is not quite in commercial production yet, but it is scheduled to be introduced in mid-2009. This product is CREE's silicon carbide (SiC) MOSFET, which is an R&D breakthrough product being sampled by specific system manufacturers.
Several manufacturers have made SiC diodes, but CREE is the first to come up with a viable MOSFET. The ability to make these parts rests on the gate structure, which requires a physics and chemistry solution. The company still has some “tweaking” to do with the process, but it appears to be well ahead of the other companies that have ventured into this technology.
CREE also has worked with gallium nitride (GaN), but Applications Engineer Manager Michael O'Neill believes SiC is a better solution for high-voltage, high-temperature MOSFETs. CREE appears to be the leader in SiC products, because it also uses that technology for diodes and LEDs.
The commercial production of 1200-V SiC power MOSFETs is now feasible because of recent advances in substrate quality, improvements in epitaxy, optimized device design, advances in increasing channel mobility with nitridation annealing, as well as the optimization of device-fabrication processes. SiC is a better power semiconductor than silicon (Si), because SiC has a much higher electric field breakdown capability (almost 10x), higher thermal conductivity and higher temperature operation capability (wide electronic band gap).
SiC excels over Si as a semiconductor material in 600 V and higher-rated breakdown voltage devices. SiC Schottky diodes at 600-V and 1200-V ratings are commercially available today, and they are already accepted as the best solution for efficiency improvement in boost-converter topologies, as well as in solar inverters by substituting them for the previously used Si PiN free-wheeling diodes that have significant switching losses
The SiC MOSFET being discussed here is a 1200-V, 20-A device from CREE that has a 100 mΩ RDSON at a +15-V gate-source voltage. Besides the inherent reduction in on-resistance, SiC also offers a substantially reduced on-resistance variation over operating temperature. From 25°C to 150°C, SiC variations are in the range of 20% versus 200% to 300% for Si. The SiC MOSFET die is capable of operation at junction temperatures greater than 200°C, but for this particular example, it is limited by its TO-247 plastic package to 150°C.
SiC technology also shows significant improvements for 1200-V devices. The performance improvement becomes even greater when compared with Si power switches rated at 6.5 kV and above.
The new SiC device exhibits a drain-source forward voltage drop of only 4.1 V, while conducting full-rated 10-A drain current with a +20-V gate-source voltage. This is equivalent to a specific on-resistance characteristic of 127 mΩ/cm2. The drain-source leakage current measured 124 nA at a 10-kV blocking voltage. In a direct comparison with a standard 6.5-kV Si IGBT in a clamped inductive switching test fixture, the SiC MOSFET exhibits 1/200th of the total switching energy of the IGBT. The SiC MOSFET's turn-on delay time is only 94 ns, compared with 1.4 µs for the IGBT, and the SiC turn-off time is only 50 ns, compared with the IGBT's 540 ns.
The switching losses of the SiC device are less than half those of the Si IGBT (1.14 mJ versus 2.6 mJ). Coupling this switching loss reduction with lower overall conduction losses, the SiC MOSFET is more efficient device for power-conversion purposes in any high-power system.
When compared with a Si IGBT, the SiC MOSFET has a substantial advantage in conduction losses, particularly at lower power outputs. By virtue of its unipolar nature that exhibits no tail currents at turn-off, there are reduced turn-off losses, as seen in the figure.