• Peter 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.

This week, I shared a new website and article that I had been working on at Warwick, entitled How can electronics engineers help to power the green transition?. Along with an accompanying cartoon, this work is aimed at encouraging young adults into power electronics, and electrical engineering more generally. This has been a labour of love that I have been working on over the last 6 months, put together by Futurum Careers, who produce free online resources and magazines aimed at encouraging 14-19-year-olds worldwide to pursue careers in science, technology, engineering, maths and medicine (#STEM).


Apart from the glamour of finally getting my own avatar (the slim, handsome chap pictured), I am delighted with the outcome of this, and I hope it will reach many aspiring young engineers out there.


A SiC transistor is a power electronic device that is revolutionising electric vehicles today, and may one day soon help reduce waste in the generation and transport of energy. For the Futurum project, I put together a water and dam analogy, an idea inspired by conversations with Jed Dorsheimer at Canaccord Genuity. The analogy attempts 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 the levee breaks! So here is a (slightly!) deeper explanation of the analogy…


An Alpine Power Converter?


Ok picture the scene – or look at the image below! – 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 water 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 much less picturesque, scene inside the drivetrain inverter of an electric vehicle, the ‘dam’ is actually a transistor, holding back a reservoir of energy stored in a battery. A current is synonymous with both examples, yet of course in the electrical version it is a flow of electrons (well actually it’s not, but I’m not going there today!) rather than a flow of water. The strength of the dam also parallels to our transistors. While the dam must be strong enough to withhold a large force from the amassed water behind it, our transistors must withstand the battery voltage – 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 the 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 too the active region of 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 (more or less…) holds true in the semiconductor, whose resistance scales with semiconductor length, and so the now shorter SiC semiconductor “pipe” 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. Even leaving these facts aside – 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 [semiconductor physics], then the levee’s going to break.

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