Researchers add 'fins' to create 1200V GaN chips
High voltage vertical transistor design could drastically reduce energy waste
Researchers from MIT, semiconductor company IQE, Columbia University, IBM, and the Singapore-MIT Alliance for Research and Technology, have presented a new vertical transistor design that, in tests, enabled GaN power devices to handle voltages of 1200V.
While the researchers emphasise that their device is a first prototype manufactured in an academic lab, they believe that further work can boost its capacity to the 3,300-to-5,000V range, to bring the efficiencies of GaN to the power electronics in the electrical grid itself.
"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 GaN 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, 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."
Narrowing the field
Although their advantages are well-known, vertical devices have been difficult to fabricate in GaN. In a transistor, a charge applied to the 'gate' switches the semiconductor material 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 GaN to direct current into a channel beneath the gate.
But the barriers are built from a material that's costly and difficult to produce, and integrating it with the surrounding GaN in a way that doesn't disrupt the transistor's electronic properties has also been tricky.
Palacios and his collaborators' alternative was to simply use a narrower device. Their vertical GaN 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.
"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."
