Last updated January 9, 2018 at 4:25 pm
A new design of transistors could dramatically cut energy waste in electric vehicles, data centres, and the power grid, according to a multi-national group of researchers.
MIT postdoc Yuhao Zhang, handles a wafer with hundreds of vertical gallium nitride power devices. Courtesy of Yuhao Zhang
Power conversion is intrinsically inefficient, a power converter will never output quite as much power as it takes in which has many implications – lost money, overheating issues and fire risks to name a few.
Improvements using gallium nitride-based power converters rather than silicon-based power converters have allowed for higher efficiency and smaller sizes.
Power electronics depend on transistors, devices in which a charge applied to a “gate” switches a semiconductor material — such as silicon or gallium nitride — between a conductive and a nonconductive state. For that 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 gallium nitride devices these have been difficult to build, until now.
At the Institute of Electrical and Electronics Engineers’ International Electron Devices Meeting this week, a group of researchers announced they have created a power converter with a redesigned vertical gallium nitride transistor, which is more efficient and can work for household and commercial purposes.
Gallium nitride is already used in power converters but they are not used in a vertical configuration.
Tomás Palacios from MIT, who worked on the new prototype said, “All the devices that are commercially available are what are called lateral devices, 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.”
The use of gallium nitride in this lateral manner has restricted it to household appliances as they can only handle voltages up to about 600 volts.
The new prototype uses a new structure that doubles that capacity to 1200 volts, which would be sufficient to use in an electric car. The team is eventually aiming for 3300 to 5000 volts, which would allow them to be reliably used in grid power networks.
Vertical devices, he says, are much better at managing heat loss, which is wasted energy.
“Vertical devices are much better in terms of how much voltage they can manage and how much current they control.”
“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.”
Applying gallium nitride transistors in a vertical device has been difficult to date.
Transistors need a small area so that the current flowing through the semiconductor material (in this case gallium nitride) can switch efficiently between a conductive and a nonconductive state.
Rather than using an internal physical barrier to route current into a narrow region of a larger device, which could be costly and interfere with the transistor itself, they simply use a narrower device and created “fins” on the transistors.
The vertical gallium nitride transistors have bladelike protrusions on top, known as “fins.” The narrowness of the fin ensures that the gate electrode will be able to switch the transistor on and off.
On both sides of each fin are electrical contacts that together act as a gate. The electrical current enters these thin fins through the top and exits through the bottom. The narrowness of the fin ensures that the gate electrode will be able to switch the transistor on and off. Making the fin so thin allows it to be far more efficient at controlling the current than previous solutions and avoids having to use multiple materials.
The team will be continuing to work on their design to reach their high-voltage targets. Making power use more efficient not only at the small scale appliances level but also at a larger power grid scale could really make a difference.
Read more on the MIT News website.
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