A new benchmark for AlGaN-on-sapphire HEMTs
Engineers from the University of Wisconsin-Madison are claiming to have produced passivated AlGaN-on-sapphire HEMTs that combine impressive breakdown fields with a record Baliga’s figure of merit.
According to the team, their field-plated Al0.6Ga0.4N HEMTs can withstand up to 2.33 kV, and have a Baliga’s figure of merit of 654 MW cm-2. This latter metric, calculated by squaring the breakdown voltage and dividing that figure by the on-resistance, is more than three times higher than the previous best for AlGaN-on-sapphire HEMTs.
The engineers point out that the performance of the latest HEMTs even surpasses that of their previous devices produced on bulk AlN. Success is attributed to careful electric-field engineering.
When discussing the pros and cons of different foundations for the HEMT, spokesman for the team, Khush Gohel, remarked: “Bulk AlN substrates can provide better thermal conductivity and a higher-quality epitaxial structure, but the lack of large-diameter substrates – 6-inch and 8-inch – makes AlN unsuitable for scaling the technology in the near future. Substrate cost is a particularly strong driver in the power semiconductor industry, which is highly cost-sensitive, so for now, the gain from AlN doesn’t justify the cost.”
Gohel and co-workers produced their devices by loading AlN-on-sapphire templates into an MOCVD reactor and depositing an AlGaN heterostructure with an n-doped Al0.81Ga0.19N barrier, included to increase the electron concentration in the two-dimensional electron gas channel (see Figure for details of the epistructure).
Fabrication of the HEMTs included metal deposition, etching, passivation and the addition of field plates. These transistors have a gate length of 2 µm, a source-to-gate spacing of 1.5 µm, and a device width of 100 µm. Gate-to-drain distances varied from 5.5 µm to 10.3 µm, the length of the first field plate ranged from 0.5 µm to 2.0 µm, and the source field plate dimensions varied from 1.0 µm to 2.5 µm.
According to Gohel, on-state characteristics of the team’s HEMTs are approaching the performance of GaN-based transistors. But they are not quite there yet, mainly due to lower electron mobility in AlGaN and the difficulty of achieving a high average electric field in the gate-drain access region.
For devices with a breakdown voltage more than 2 kV, on-resistance is 8.3 mΩ cm². Commenting on the latter, Gohel remarked: “I believe this can be improved significantly by reducing channel sheet resistance and improving field spreading, which would allow us to block the same voltage with a smaller gate-to-drain spacing.”
When measuring breakdown voltage, the team immersed their HEMTs in Flourinert FC-40 to suppress premature air breakdown. Higher breakdown fields resulted from the addition of two field plates, which are claimed to spread the field in the gate drain access region.
A HEMT with a 1 µm gate field plate and a 2.5 µm source field plate produced the most impressive results – a breakdown voltage of 2334 V and an average breakdown field of 2.26 MV cm-1.
Gohel says that as the critical field for Al0.6Ga0.4N exceeds 10 MV cm-1, there’s “clear room” to improve the spreading of the electric field in the gate-drain access region.
“As for passivation, we’re still investigating its role, along with material quality and processing effects, since several factors beyond the field plate design likely limit the breakdown field, and we don’t yet have a complete picture of their individual contributions.”
Plans for the team include a further reduction in on-resistance, while simultaneously optimising the electric field in the gate-drain access region. “We also want to shift focus toward the device’s dynamic performance, rather than just its static/DC characteristics.”
Reference
K.Gohel et al. Appl.Phys.Express 19 066502 (2026)
