A new gallium oxide transistor can withstand voltages of over 8,000 volts (V), the highest ever reported for a device of comparable size. The advance opens up exciting possibilities for compact, energy-efficient power electronics systems based on a technology that is only eight years old: the first gallium oxide transistors were reported in 2012.
“Those are extraordinary numbers compared to what’s reported,” says Uttam Singisetti, a professor of electrical engineering at the University of Buffalo who led the new device research published in IEEE Electron Device Letters. “Reaching 8kV in eight years is a big achievement.”
Power electronics systems are crucial in electric vehicles, trains, electric aircraft, and the inverters that deliver energy from solar and wind arrays to the grid. These systems control and manipulate power instead of digital data, switching between AC and DC, regulating voltage levels, and managing power flow. The transistors used in these systems need to handle high voltages and temperatures. And the thinner a device, the lower its resistance, making it more energy-efficient, which translates to a less wasteful grid or a vehicle that can travel farther on a charge.
Silicon carbide has started to replace silicon in power electronics. Its advantage over silicon? Its bandgap, the energy needed to excite electrons into the conduction band, is 3.4eV, about three times higher than silicon’s 1.1eV. This lets silicon carbide transistors withstand far higher voltages and temperatures than silicon devices. But their commercial success has been limited by their high price.
Gallium oxide, it turns out, beats silicon carbide with its nearly 5eV bandgap. Researchers have been trying to make high-voltage gallium oxide transistors but the devices have so far had relatively modest breakdown voltages, the voltage at which the semiconductor material switches from insulator to conductor. The higher this voltage, the more power a device can handle.
Singisetti and his colleagues carefully analyzed the devices they made and found that the breakdown wasn’t happening in the gallium oxide layer. Instead it was the thin air layer above the material that was breaking down. So the researchers decided to coat the material with a strong polymer. This simple protective cap led to devices that could handle over 8,000 V.
The new gallium oxide transistors are 80 microns in length. By comparison, today’s silicon carbide transistors can carry 10,000 V in a device around 150 microns long.
But gallium oxide isn’t quite ready to break out of research laboratories yet. “One of the biggest challenges with gallium oxide is its low thermal conductivity,” says Singisetti. “When the device operates it gets very hot and we would need innovative thermal management.”
Researchers are thinking about ways around this problem. One possible solution, says Singisetti, is to make gallium oxide devices on a thin silicon carbide substrate to take advantage of silicon carbide’s high thermal conductivity.