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3 June 2020

Buffalo reports gallium oxide transistor with breakdown exceeding 8kV

In research supported by the US Air Force Office of Scientific Research and by the US National Science Foundation (NSF), researchers in the University of Buffalo’s Department of Electrical Engineering have developed a gallium oxide (β-Ga2O3) metal-oxide-semiconductor field-effect transistor that can handle voltages of more than 8000V (Shivam Sharma et al, ‘Field-Plated Lateral Ga2O3 MOSFETs With Polymer Passivation and 8.03 kV Breakdown Voltage’, IEEE Electron Device Letters, vol41, issue6 (June 2020), p836).

Associate professor Uttam Singisetti (senior author of the paper) and students in his lab have been studying the potential of gallium oxide, including previous work exploring transistors made from the material.

Gallium oxide has an ultrawide bandgap energy of about 4.8eV. This exceeds that of the incumbent power electronics material silicon (1.1eV) as well as its potential replacements including silicon carbide (about 3.4eV) and gallium nitride (about 3.3eV). Systems made with wide-bandgap materials can be thinner, lighter and handle more power than systems made of materials with narrower bandgaps.

A key innovation in the new transistor revolves around the passivation process (i.e. coating the device to reduce the chemical reactivity of its surface), for which a layer of SU-8 epoxy-based polymer was added.

Tests conducted in March - for MOSFETs with a gate-drain length (Lgd) ranging from 30μm to 70μm and across two process runs - showed consistently higher breakdown voltages in passivated devices compared with non-passivated devices. The tests also showed that the transistor can handle a maximum voltage of 8032V before breaking down (for Lgd up to 70μm), which is more than similarly designed transistors made of silicon carbide (SiC) or gallium nitride (GaN) that are under development.

“The higher the breakdown voltage, the more power a device can handle,” says Singisetti. “The passivation layer is a simple, efficient and cost-effective way to boost the performance of gallium oxide transistors.”

Simulations suggest a figure of more than 10MV/cm for field strength (the intensity of an electromagnetic wave at a given spot, which eventually determines the size and weight of power electronics systems). “These simulated field strengths are impressive. However, they need to be verified by direct experimental measurements,” Singisetti says.

The transistor could lead to smaller and more efficient electronic systems that control and convert electric power in electric cars, locomotives and airplanes, improving their range.

“To really push these technologies into the future, we need next-generation electronic components that can handle greater power loads without increasing the size of power electronics systems,” says senior author Uttam Singisetti, adding that the transistor could also benefit microgrid technologies and solid-state transformers.

Tags: Gallium oxide transistors MOSFET