MOSFET

Another device, not yet mentioned is the IGBT, Isolated Gate Bipolar Transistor. I have NO idea if there are audio applications here.

Just FYI, the older Thresholds (T200, T100, T400, etc from that era) & several of the down-market Forte amps also built at the same time used IGBTs in their output stages. The sound of these class-A amps & the Forte class-A, class-AB amps is quite good. I find it a bit confined compared to my class-A BJT amp but this is just 1 amp compared against my own.

Bombaywalla - This is a very strange application since IGBTs are used mostly for very fast switching (motor control, switching power supplies etc.). Do you know by any chance why they were used in class A or AB audio amps?

Kijanki,
I am NOT an engineer. I MAKE the darn things in a wafer fab. The HexFet is a pretty neat design, and i guess you'd say it IS vertical. The gate and source regions are on the top while the drain is the bottom. Yes, one of the selling points is apparently RDON, which is Resistance of the Device ON. I suspect this plays directly into damping factor and maybe why Carver used our stuff.
I, too, am a fan of 'd' amps, having a PSAudio GCC series integrated, using the B&O ASP modules. I have an International Rectifier stereo amp out in the garage waiting for me to figure out how to build a power supply. I could take the chickens way out and buy Gel Cells for a brute force DC/Battery supply.
Again, I am not an engineer but I suspect anything that switches quickly enough could be used as an amp's output stage.....RE:: IGBTs. These devices, at least as I see them are usually higher voltages, maybe 600v.