CMOS technology that works above 300 °C
Honeywell SRAM HT6256.

The RAM provides guaranteed performance over the full
-55 to +225°C temperature range. Typically, parts will operate up to +300°C for a year, with derated performance.

But I feel that this technology protected by many NDAs.
The &quot;Prolonged 500 °C Operation of 100+ Transistor Silicon Carbide
Integrated Circuits&quot; article tells that:

This report describes more than 5000 hours of successful 500 °C
operation of semiconductor integrated circuits (ICs) with more than
100 transistors.

&quot;Ozark Integrated Circuits&quot; reports that they can produce 600°C ICs with SiC transistors on aluminium substrate.
Also you may look at the &quot;Honeywell's HTMOS™ High Temperature Electronics&quot; article and the &quot;Geothermal Technologies Program 2010&quot; article.