However, commercial gallium nitride power devices can’t handle voltages above about 600 volts which limits their use to household electronics.
Now, researchers from MIT, semiconductor company IQE, Columbia University, IBM, and the Singapore-MIT Alliance for Research and Technology, have come up with a new design that, in tests, enabled gallium nitride power devices to handle voltages of 1,200 volts, according to MIT.
The new device uses a fundamentally different design from existing gallium nitride power electronics.
“All the devices that are commercially available are what are 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.”
For one thing, Palacios explains, current flows into one surface of a vertical device and out the other. That means that there’s simply more space in which to attach input and output wires, which enables higher current loads.
For another, Palacios says, “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 their advantages are well-known, vertical devices have been difficult to fabricate in gallium nitride. 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 the past, researchers had attempted to build vertical transistors by embedding physical barriers in the gallium nitride to direct current into a channel beneath the gate.
Palacios and his collaborators adopted a simple but effective alternative. Rather than using an internal barrier to route current they simply use a narrower device. Their vertical gallium nitride transistors have bladelike protrusions on top, known as “fins.” On both sides of each fin are electrical contacts that together act as a gate. Current enters the transistor 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.
The researchers say their device is just a first prototype but they believe 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.
The team includes first authors Yuhao Zhang, a postdoc in Palacios’s lab, and Min Sun, who received his MIT PhD in the Department of Electrical Engineering and Computer Science (EECS) last spring; Daniel Piedra and Yuxuan Lin, MIT graduate students in EECS; Jie Hu, a postdoc in Palacios’s group; Zhihong Liu of the Singapore-MIT Alliance for Research and Technology; Xiang Gao of IQE; and Columbia’s Ken Shepard.
Photo: MIT postdoc Yuhao Zhang, handles a wafer with hundreds of vertical gallium nitride power devices fabricated from the Microsystems Technology Laboratories production line.
Courtesy of Yuhao Zhang
