5-3-2 High-Power Device Operation

5-3-2 High-Power Device Operation

The high breakdown field and high thermal conductivity of SiC coupled with high operational junction

temperatures theoretically permit extremely high-power densities and efficiencies to be realized in SiC

devices. The high breakdown field of SiC relative to silicon enables the blocking voltage region of a

power device to be roughly 10×thinner and 10×heavier doped, permitting a roughly 100-fold

beneficial decrease in the blocking region resistance at the same voltage rating. Significant energy

losses in many silicon high-power system circuits, particularly hard-switching motor drive and power

conversion circuits, arise from semiconductor switching energy loss . While the physics of

semiconductor device switching loss are discussed in detail elsewhere, switching energy loss is

often a function of the turn-off time of the semiconductor switching device, generally defined as the

time lapse between application of a turn-off bias and the time when the device actually cuts off most

of the current flow. In general, the faster a device turns off, the smaller its energy loss in a switched

power conversion circuit. For device-topology reasons discussed in References 3,8, and 19–21, SiC’s

high breakdown field and wide energy bandgap enable much faster power switching than is possible

in comparably volt–ampere-rated silicon power-switching devices. The fact that high-voltage operation

is achieved with much thinner blocking regions using SiC enables much faster switching (for comparable

voltage rating) in both unipolar and bipolar power device structures. Therefore, SiC-based power

converters could operate at higher switching frequencies with much greater efficiency (i.e., less switching

energy loss). Higher switching frequency in power converters is highly desirable because it

permits use of smaller capacitors, inductors, and transformers, which in turn can greatly reduce overall

power converter size, weight, and cost.

While SiC’s smaller on-resistance and faster switching helps minimize energy loss and heat generation,

SiC’s higher thermal conductivity enables more efficient removal of waste heat energy from the active

device. Because heat energy radiation efficiency increases greatly with increasing temperature difference

between the device and the cooling ambient, SiC’s ability to operate at high junction temperatures permits

much more efficient cooling to take place, so that heat sinks and other device-cooling hardware (i.e., fan

cooling, liquid cooling, air conditioning, heat radiators, etc.) typically needed to keep high-power devices

from overheating can be made much smaller or even eliminated.

While the preceding discussion focused on high-power switching for power conversion, many of the

same arguments can be applied to devices used to generate and amplify RF signals used in radar and

communications applications. In particular, the high breakdown voltage and high thermal conductivity

coupled with high carrier saturation velocity allow SiC microwave devices to handle much higher power

densities than their silicon or GaAs RF counterparts, despite SiC’s disadvantage in low-field carrier

mobility.

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