Here's a chunky TO247 transistor clamped to a heat sink. The insulator is a 20 mil thick slab of aluminum nitride, a sample from Sienna Technologies. Silicone grease on both sides of the AlN.
The fet is rated for 446 watts and spec'd 0.28 K/W theta J-C, and, at40 watts dissipation, I'm measuring 0.2 K/W from the top of the package to the surface of the heat sink.
They can supply it in 13 mils thickness, too.