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  • Writer's picturePeter Gammon

When the levee breaks! A useful water analogy for SiC power devices.

Take a splash in SiC waters, as PGC dissect a water analogy that can be used to grasp the fundamental benefits of SiC. We review the concept of using dams as transistors and explain the limits of such an analogy. Exercises like this may not satisfy purists, but we consider that they are helpful in disseminating these deep concepts to wider audiences.


SiC is a semiconductor that is finding a home in electric vehicles. With promises of range extension, or lighter, cheaper battery packs, the majority of major car manufacturers are developing SiC power converters to replace legacy silicon-based products.


The fundamentals of why this semiconductor is complicated, talk of its high breakdown electric field and wide bandgap enough to put off our undergraduate students, let alone investors or other non-specialists trying to understand this field.


For this reason, I resorted to an age-old water analogy in an attempt to condense the physics behind the benefits of SiC into a concept understandable by those without a degree in Engineering or Physics. It draws parallels between the flow of electricity in a power converter, as controlled by transistors, with the flow of water controlled by a dam. Specifically, I use it in the article to outline the concept behind the immense voltage-blocking capability of SiC.


However, pun completely intended, it is an analogy that can only be stretched so far before bursting – and you’ll see below where indeed, the levee breaks.



An Alpine Power Converter?


Picture the scene, of a reservoir, a great energy reserve in the mountains, from which two waterways – rivers or tributaries – are flowing. Two dams control the flow of flow into these from the reservoir. Each is strong enough to hold back all the water stored in the dam and they both have a gate that can be used to release water downstream.


In a parallel but less picturesque scene inside an electric vehicle, a "reservoir" of energy is stored in a battery and numerous transistors control its flow to the rest of the system. A "current" is synonymous with both examples, yet of course in the electrical version it is a flow of electrons rather than a flow of water.


In both scenarios, one can also consider the 'strength' of the dam/transistor. While the dam must be strong enough to withhold a large force from the amassed water behind it, our transistors must withstand a potential energy, the battery voltage of 400 or 800 volts for example, in today’s electric vehicles.


Returning to the mountains. If you look closer at the two dams, it becomes obvious that the old dam on the left is significantly thicker than the new, slimline dam on the right. This is because the new dam was built using a modern, reinforced concrete, with a strength nine times greater than the old concrete material of yesteryear. It meant that the new dam could be vastly slimmed down compared to the old one.


The underpinning benefit of wide bandgap materials is their relative ‘strength’ compared to silicon, in withstanding a voltage. Known as the critical electric field of a semiconductor and measured in volts per centimetre, a unit length of SiC material can hold back 9x more voltage than Si before it goes bang! All SiC’s other benefits (its great thermal conductivity, wide bandgap etc), are a fantastic bonus that add to its appeal, but this critical electric field is key.


In both scenarios, the increased “strength” can be exploited in one of two ways. Replacing the old Si concrete dam with the same amount of SiC reinforced concrete results in a new dam could hold back 9x the water pressure and it could therefore be utilised to control even greater reservoirs. Or else, in the same location, blocking the same body of water, the new SiC dam can be slimmed down making it 9x thinner than the original. So the parallel continues to the transistor, whose current carrying ‘drift region’ is 9x thinner than the silicon transistor it is replacing. This is the case now as 600 V and 1200 V SiC MOSFETs begin to replace Si IGBTs of the same voltage class in electric vehicle inverters.


Returning to our mountain scene for a final time. Dams fully constructed, water is released through them via sluiceways – pipes that traverse the full thickness of the concrete. The flow of water through a pipe decreases proportionally with its length (I had to look it up but its true!) and so the flow rate through the new dam is 9 times greater than through the old dam. Once again this holds true in the semiconductor (more or less…), whose resistance scales with semiconductor length. Hence, the now shorter SiC semiconductor “pipe” has less resistance and allows significantly more electrical current to pass through.


When the Levee Breaks?

A postscript is necessary to this to point out the flaws in the analogy, the point beyond which my imagination... runs dry. The parallels drawn between scaling down a dam/transistor and its benefits on resistance hold, though implicitly we are comparing 'devices' of the same type and the same voltage rating. Specifically with charge moving in only one direction, these are unipolar devices, MOSFETs. Yet, if this were true, then the resistance reduction of a fully optimised SiC MOSFET would be approximately 500x lower (not 9x) than the Si MOSFET, as the analogy cannot account for the greater doping of the SiC drift region (water pipes of greater radius in the new dam for some reason?!).


However, SiC MOSFETs aren’t replacing Si MOSFETs, they are replacing Si IGBTs, bipolar devices with charge movement in both directions, simultaneously. This is a stretch though for water, and I can conjure up no way of accounting for bipolar water, specifically the flow of holes in the direction opposite to electrons. This would have to be an anti-water-molecule, pushed the wrong way through the dam – and I don’t think air fits the bill. This means that the fast switching speeds possible with the SiC MOSFET, compared to the Si IGBT, which has such a profound effect on the size reduction of a SiC inverter, is also beyond this analogy.


Finally, I would like to see either dam work at >10 kHz!


Conclusion

These systems level comparisons are fun, and although inevitably flawed, I have found that they help to deliver at least a surface level of understanding of complex semiconductor and device physics to audiences who have not sat through years of device physics in university. Within the University it is something we might ask Undergraduates to try as they require a certain depth of knowledge to do well. Ultimately, however, these are abstract ideas and hence if it keeps on raining then the levee’s going to break.

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