It’s heady times in power electronics. After decades of domination by silicon, two newer materials—silicon carbide and gallium nitride—have begun taking over multibillion-dollar markets. Silicon carbide is now the semiconductor of choice for the inverters and chargers in electric vehicles, for example. And if you’ve purchased a wall charger lately for your smartphone or laptop, chances are good that it uses gallium nitride.
The newer materials, known as wide-bandgap semiconductors, are taking over these and other power-electronics applications because they offer many superior characteristics. And yet wide-bandgap technologies still have fundamental weaknesses. For a silicon-carbide transistor, a big one is relatively low mobility of electrons in the channel—the area under the device’s gate through which current flows between the source and the drain. That low mobility prevents SiC transistors from switching at high rates. That, in turn, limits their efficiency in applications such as converting between alternating current and direct current. Gallium-nitride transistors, on the other hand, have a quirk known as “dynamic on-resistance,” which means that when the device is conducting current, the resistance of the device depends on the voltage—higher voltage means higher on-resistance. Another problem with GaN is that the physical size of the device, and therefore its cost, goes up as its voltage-blocking capability does, a key ability for devices expected to turn on and off voltages that are many times higher than those found inside, say, a typical computer.
What if you could combine GaN and SiC in a single device that minimizes the weaknesses of each and maximizes their strengths? That’s the question that drove a team of 16 researchers at the Hong Kong University of Science and Technology and three other institutions in China. After years of work they finally claimed success by fabricating a transistor, which they call a Hybrid Field-Effect Transistor, or HyFET. They described their work in a paper presented at the IEEE International Electron Devices Meeting, held this past December in San Francisco.
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