For thermal applications for semiconductor devices, single-crystal diamond doesn't have any significant advantage over polycrystalline diamond. Normally, a thermal conductivity of > 2000 W/mK is more than enough for most thermal management challenges that any semiconductor device represents, even the most powerful RF power amplifiers. And as is pointed out, since polycrystalline diamond can be grown in bigger wafer sizes, up to 140mm in diameter, it is more cost effective today than single-crystal diamond.
CVD diamond is okay, but it is usually not monocrystaline. Some of the advances that are being made now, though, will hopefully get us to wafer-sized monocrystaline diamonds in the near future. I know there's at least one ompany making these in smaller wafer sizes up in NY, somewhere...
Yes, the diamond used is synthetic diamond made using microwave Chemical Vapor Deposition (CVD). It can be made in many different Thermal and Optical grades. For Thermal grades, Element Six offers 5 different grades with thermal conductivity ranging from 1000 W/mK to 2000 W/mK.
My first computer, one of the "portable" things that was almost too heavy to carry. My wife (fiancee at the time) was writing a paper that was due at 5:00 on a Friday. At 2:00, she called in a panic "The computer is doing crazy. I type this key and a different letter comes up." I was at work at the time and couldn't help so she pulled out a typewriter to finish typing the paper.
When I arrived, I immediately spotted the problem. She had blocked the vent with a book.
If that ptoblem happened today, no typewriter but we have several computers. That's fine as long as she's not typing in Wordperfect 11, her preference.
More recently, I have numerous BR30 LED lamps in recessed lighting fixtures. One day, one of them went out. A few minutes later, it came back on, then went out again after a few more minutes. Suspecting a heat problem, I removed the bulb from the fixture and installed it in a lamp, just to try it. The LED stayed on all day. The air around it dissipated enough heat to keep it on. Reinstalling it in the recessed fixture caused the problem to return.
One of these days, I'll try to isolate the problem using a blow dryer. It should make a good blog topic.