Western North America (WNA) experienced an unprecedented heatwave from late June to early July 2021, which lies far outside the historical range. Based on the model outputs from Coupled Model Intercomparison Project Phase 6, we find that the likelihood of a heatwave over WNA similar to the 2021 one increases with global warming. Such a heatwave is projected to occur more frequently with increased extreme temperature and shortened return period. It means that a rare event in the current climate will be a more common event in a warmer climate, especially under a high‐emission scenario like the Shared Socioeconomic Pathways 585 (SSP5‐8.5). Moreover, we show a large expansion of areas over WNA that will break the 2021 record in the future with an increasing level of emission scenario. Nevertheless, some heatwave records west of the Rocky Mountains are still difficult to break, highlighting the specific extremity of the 2021 WNA heatwave.

This paper reports on the fabrication and characterization of gate-last self-aligned in situ SiN x /AlN/GaN MISHEMTs. The devices featured in situ grown SiN x by metal-organic chemical vapor deposition as a gate dielectric and for surface passivation. Selective source/drain regrowth was incorporated to reduce contact resistance. SiN x sidewall spacers and low-κ benzocyclobutene polymer (κ = 2.65) supporting layers were employed under the gate head to minimize the parasitic capacitance for high-frequency operation. The device with a gate length (L G ) of 0.23 μm exhibited a maximum drain current density (I DS ) exceeding 1600 mA/mm with a high ON/OFF ratio (I ON /I OFF ) of over 10 7 . The current gain cutoff frequency ( f T ) and maximum oscillation frequency ( f max ) were 55 and 86 GHz, respectively. In addition, the effect of temperature, from room temperature up to 550 K, on the dc and RF performances of the gate-last self-aligned MISHEMTs was studied.Index Terms-AlN/GaN, benzocyclobutene (BCB) planarization, gate-last self-aligned, high-temperature, in situ SiN x , MISHEMT, RF, source/drain (S/D) regrowth.

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