Power electronics are everywhere. They do things like modify voltages or convert between direct and alternating currents. They are in the power bricks that are used to charge portable devices, battery packs of electric cars and power grids. In power grids, power electronics mediate between high-voltage transmission lines and the lower voltages of household electrical sockets.
Power conversion is inefficient. A power converter will never output as much power as it takes in. But recently, power converters made from gallium nitride have begun to reach the market, claiming they have higher efficiencies and smaller sizes than conventional, silicon-based power converters.
But commercial gallium nitride power devices can’t handle voltages above 600 volts, which limits their use to household electronics.
At the Institute of Electrical and Electronics Engineers’ International Electron Devices Meeting this week, researchers from MIT, IQE, Columbia University, IBM and the Singapore-MIT Alliance for Research and Technology, presented a new design that enabled gallium nitride power devices to handle voltages of 1,200 volts in testing.
That is enough capacity for use in electric cars, but the researchers emphasize that their devices are a first prototype that was manufactured in a lab. They believe that further work can boost its capacity to the 3,300-to-5,000-volt range, to bring the efficiencies of gallium nitride to the power electronics in the electrical grid itself.
This is because the new device uses a different design from existing gallium nitride power electronics.
"All the devices that are commercially available are what is called lateral devices," says Tomás Palacios, who is an MIT professor of electrical engineering and computer science, a member of the Microsystems Technology Laboratories, and senior author on the new paper. "So the entire device is fabricated on the top surface of the gallium nitride wafer, which is good for low-power applications like the laptop charger. But for medium- and high-power applications, vertical devices are much better. These are devices where the current, instead of flowing through the surface of the semiconductor, flows through the wafer, across the semiconductor. Vertical devices are much better in terms of how much voltage they can manage and how much current they control."
For one thing, current flows into one surface of a vertical device and out of the other. This means that there is simply more space to attach input and output wires, which enables higher current loads.
According to Palacios, “When you have lateral devices, all the current flows through a very narrow slab of material close to the surface. We are talking about a slab of material that could be just 50 nanometers in thickness. So all the current goes through there, and all the heat is being generated in that very narrow region, so it gets really, really, really hot. In a vertical device, the current flows through the entire wafer, so the heat dissipation is much more uniform."
Although the advantages are well-known, the vertical devices have been difficult to fabricate in gallium nitride. Power electronics depend on transistors, devices in which a charge is applied to a “gate” switches a semiconductor material between conductive and a nonconductive state.
For the switching to be efficient, the current flowing through the semiconductor needs to be confined to a relatively small area, where the gate’s electric field can exert an influence on it. In the past, researchers have attempted to build vertical transistors by embedding physical barriers in the gallium nitride to direct current into a channel beneath the gate.
But the barriers are built from a temperamental material that is costly and difficult to produce. Integrating it with the surrounding gallium nitride in a way that doesn’t disrupt the transistor’s electronic properties has been challenging.
Palacios and his collaborators adopt a simple and effective alternative. The team includes Yuhao Zang, a postdoc in Palacios’ lab, Min Sun, who received his MIT Ph.D. in the Department of Electrical Engineering and Computer Science (EECS), Daniel Piedra and Yuxuan Lin, MIT graduate students in EECS, Jie Hu, a postdoc in Palacios’ group, Zhihong Liu of the Singapore-MIT Alliance for Research and Technology, Xian Gao of IQE and Ken Shepard from Columbia.
Instead of using an internal barrier to route current into a narrow region of a larger device, the research team just used a narrower device. Their vertical gallium nitride transistors have bladelike protrusions on the top called “fins.” On both sides of each fin are electrical contacts that act as a gate. Current enters the transistors through another contact, on top of the fin, and exits through the bottom of the device. The narrowness of the fin ensures that the gate electrode will be able to switch the transistor on and off.
"Yuhao and Min's brilliant idea, I think, was to say, 'Instead of confining the current by having multiple materials in the same wafer, let's confine it geometrically by removing the material from those regions where we don't want the current to flow,'" Palacios says. "Instead of doing the complicated zigzag path for the current in conventional vertical transistors, let's change the geometry of the transistor completely."