Very high temperature (>500C) IC packages
Semiconductors become conductors above a temperature related to the band-gap. A few materials, such as pure carbon (e.g., diamond) and silicon carbide (SiC), have been used to make experimental or custom designed devices such as low power IC's and high power IC's that operate at 500°C.

As far as thermal transmission and insulation, diamond is ne plusa ultra. Since gallium wets diamond, you could make a Ga/C/Ga sandwich to conduct heat and insulate electrically. The diamond probably would be less expensive than the special-purpose IC!

Though you might be able to order (and afford!) such a device, might it be possible to move signal processing to a cooler location? Admittedly, in borehole logging and spacecraft to land on Venus, this might not be a choice, but look for a more practical alternative.