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This paper reviews how high‑electron‑mobility transistors (HEMTs) made from III‑nitride materials behave at high temperatures. The authors focus mainly on gallium nitride (GaN) devices and on how heat changes their materials, transistor designs, and circuit performance. They identify which parts of the device help it survive heat and which parts are most likely to fail. They also point out a clear gap: very few devices have been shown to run for long periods at temperatures above about 400–500 °C.
HEMTs use a thin conducting sheet called a two‑dimensional electron gas, or 2DEG, that forms where two different semiconductor layers meet. That 2DEG gives high current and low leakage even at high temperature because it does not depend on dopants that break down with heat. The review examines different device styles, including normally‑on (depletion‑mode) devices and normally‑off (enhancement‑mode) designs, and several approaches to make normally‑off transistors, such as adding a p‑GaN gate layer, etching a recessed gate, implanting fluorine to add fixed negative charge, or putting an insulator under the gate (MISHEMT).
The authors summarize how specific materials and layer choices affect thermal stability. Common barrier layers include aluminum‑gallium nitride (AlxGa1−xN), indium‑aluminum nitride (InxAl1−xN), scandium‑aluminum nitride (ScxAl1−xN), and aluminum nitride (AlN). AlGaN is widely used but shows thermal instability above roughly 400 °C, with faster loss of drain current and higher resistance from strain relaxation and dislocations. InAlN can be grown to match GaN and so avoids some strain‑related failures, while ScAlN can give very strong polarization but can suffer decomposition or mismatch at higher Sc content. The review also notes practical design limits, such as keeping Al0.25Ga0.75N below about 30 nm to preserve strain during growth.
Why this matters is practical as well as technical. High‑temperature electronics are needed in hypersonic vehicles, deep oil and geothermal wells, nuclear reactors, and planetary probes. For example, electronics close to hypersonic surfaces may see internal temperatures over 500 °C or short exposure to much higher surface temperatures, and down‑hole instruments must run continuously above 250 °C for years. Higher allowed device temperatures can also shrink cooling systems because radiative cooling increases rapidly with temperature. Despite some demonstrations of devices functioning beyond 300 °C — and depletion‑mode HEMTs reported at temperatures up to 1000 °C in some tests — the authors emphasize that long‑duration reliability at the highest temperatures is largely unproven.
The review lists key remaining challenges and sources of uncertainty. Long‑term operation at high temperature is limited by material degradation, gate control and leakage, contact reliability, interface changes, and damage from thermal cycling and environment (air or nitrogen can accelerate aging). The authors highlight a “critical gap” in the literature: few demonstrations show continuous operation longer than about 1,000 hours at temperatures above 400–500 °C. In short, III‑nitride HEMTs have promising properties for extreme heat, but more work is needed to prove they can last for the long missions and harsh environments that would benefit most.