Silicon Carbide MOSFETs Challenge IGBTs

Sep 1, 2008 12:00 PM
By Michael O'Neill, Applications Engineering Manager, CREE, Durham, N.C.

In light of recent silicon carbide (SiC) technology advances, commercial production of 1200-V 4H-SiC^[1] power MOSFETs is now feasible. There have been improvements in 4H-SiC substrate quality and epitaxy, optimized device designs and fabrication processes, plus increased channel mobility with nitridation annealing.^[2] SiC is a better power semiconductor than Si, because of a 10-times higher electric-field breakdown capability, higher thermal conductivity and higher temperature operation capability due to a wide electronic bandgap.

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 are already accepted as the best solution for efficiency improvement in boost converter topologies. In addition, these diodes find use in solar inverters, because they have lower switching losses than the Si PIN freewheeling diodes now used in that application.

At 600-V and 1200-V ratings, IGBTs have been the switch of choice for power conversion. Previously, Si MOSFETs were handicapped in those applications by their high on-resistance (R_DSON). At high breakdown voltages, R_DSON increases approximately with the square of the drain-source breakdown voltage V_DSMAX.^[3] The R_DSON of a MOSFET consists of the sum of the channel resistance, the inherent JFET resistance and the drift resistance (Fig. 1). The drift resistance (R_DRIFT) is the dominant portion of the overall resistance, where d equals drift-layer thickness, q equals electron charge, ìn equals channel mobility and N_D equals doping factor.

The new generation of SiC MOSFETs cuts drift-layer thickness by nearly a factor of 10 while simultaneously enabling the doping factor to increase by the same order of magnitude. The overall effect results in a reduction of the drift resistance to 1/100th of its Si MOSFET equivalent.

The improved SiC MOSFET discussed here is an engineering sample of a 1200-V, 20-A device with a 100-mV R_DSON at a 15-V gate-source voltage. Besides its inherent reduction in on-resistance, SiC also offers a substantially reduced on-resistance variation over its operating temperature. From 25°C to 150°C, SiC variations are in the range of 20%, compared with 200% to 300% for Si. The SiC MOSFET die is capable of operation at junction temperatures greater than 200°C, but the engineering sample is limited in temperature to 150°C by its TO-247 plastic package.

Compared with a Si IGBT, a SiC MOSFET has a substantial advantage in conduction losses, particularly at lower power outputs. By virtue of its unipolar nature, it has no tail currents at turn-off, thereby leading to greatly reduced turn-off losses. Table 1 shows the switching loss difference when compared with a standard off-the-shelf 1200-V IGBT.

The switching losses of a SiC MOSFET are less than half those of a Si IGBT (1.14 mJ versus 2.6 mJ, respectively). Combining this switching-loss reduction with the lower overall conduction losses, it is clear that the SiC switch is a much more efficient device for high-power-conversion systems.

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