SOI (Silicon On Insulator)

"A 225°C Rated ASIC for Quartz Based Downhole Transducers". https://quartzdyne.com/technical-papers/ . In the paper, they say "Custom ICs designed using High Temperature rated SOI processes have been shown to function above 300°C" and they reference https://ieeexplore.ieee.org/abstract/document/5265004

Tekmos High Temperature ASICs, 250ºC and 200ºC, https://www.tekmos.com/products/high-temperature-semiconductors/high-temperature-asics-250-c-and-200-c .

"basic [SOI] building blocks measured up to 300°C are shown". https://ieeexplore.ieee.org/document/7168845 (2015)

"Extending the operation of low-cost junction-isolated CMOS to this [250C] temperature is feasible through straightforward changes in process technology, circuit styles, and layout design rules". https://ieeexplore.ieee.org/abstract/document/5265004 (1999)

"The Tekmos TK22V10H [PAL] will operate up to 300ºC". https://www.tekmos.com/products/high-temperature-semiconductors/tk22v10h-high-temperature-pal