How to cool a collector of a transistor that is capacitance sensitive?

Is it a tuning issue?

Any chance of neutralizing the capacitance?

Is the frequency high enough to make a strip line design practical - then your heat sink might be part of the circuit board or mounted to it.

Minimize it by using a small part mounted heat sink? And fan . . .

In P.A. stage in a VHF application you might use part of the output tank or filter inductance. We used to press TO18 transistors into the end of 1/4" copper tubing tank circuits to cool them . . .

Off top of head,

Hi,

it is for broadband use so the capacitance can not be tuned away by a tank circuit. Also the output/load impedance is too high (500 ohms) to include the capacitance into a microstripline. I thought about using bare chips, but the transistors are not available as chip (at least not at the quantities we need). If they were, I could maybe use thick AlN as an isolator as it has a good ratio of thermal conductance to dielectric constant (180w/mk to 8). I know only BeO (260w/mk to 6.5, toxic) and diamond (2000w/mk to 5.7, a bit expensive) with better ratios. (Do you know better/cheaper ones?) I also thought about using AlN cubes with 3x3x3mm placed directly under the position of die mount under the TO126 package, but don't know how to fix the cubes and the transistors on them. Thermal conductivity of the 3x3x3 cube would be enough and I could keep the dielectric constant of the remaining area of the package to heat sink at 1.

Any other ideas?

Greg P.S.: Would like to not use fans because of noise and maintainance issues

What's wrong with the obvious answer of sticking them in oil, anyway?