Electronic systems that control and convert electric power are critical in many applications such as electric cars. These systems need to be small and efficient in order for the electric vehicles to travel further on one charge. The ultrawide-bandgap semiconductor gallium oxide (specifically β-Ga₂O₃) is proving to be a strong contender for high-voltage electronics used in such vehicles. A research group led by Uttam Singisetti of the University of Buffalo achieved a breakdown voltage of more than 8,000 V with their field-plated lateral metal-oxide field-effect transistors (MOSFETs). The key to their success is a polymer passivation coating they applied to their device, as reported in a recent issue of IEEE Electron Device Letters.

The 4.5-4.9 eV bandgap of gallium oxide exceeds that of silicon, silicon carbide, and gallium nitride, all very popular electronic materials. The researchers fabricated their devices with an Fe-doped Ga₂O₃ substrate. A major innovation was the use of the epoxy-based polymer SU-8 as a passivation coating in order to reduce surface chemical reactivity. Their devices have a gate length of 2 µm and gate-source separation of 3 µm.

The researchers tested both non-passivated and passivated devices. While the non-passivated MOSFET shows a breakdown voltage of 2.7 kV, the passivated device with an SU-8 layer of about 10-µm thickness shows a breakdown voltage of 6.72 kV, for a gate-drain separation of 40 μm. The breakdown increases with gate-drain separation up to 70 μm, giving a maximum breakdown voltage of 8.03 kV.

“The higher the breakdown voltage, the more power a device can handle,” says Singisetti, in a news release from the University of Buffalo.

According to Singisetti, researchers are putting a lot of effort into decreasing the on-resistance of the devices and reach the theoretically predicted values. “Gallium oxide devices can also enable extremely high breakdown voltage (>20 kV), which have applications in long distance, high voltage power transmission,” he tells MRS Bulletin.  

While single devices have been demonstrated in the laboratory, the current scalability, reliability, and thermal management needs to be demonstrated before they can be used in power electronics. “It may take 5-10 years before we see real-world applications,” Singisetti says.

Future opportunities exist, though, for applications that require high voltage rating (>5 kV), according to Singisetti. “Potential application areas are electric locomotives, high-voltage transmissions, electric planes, and ships,” he says.

Read the abstract in IEEE Electron Device Letters.