MOSFET

Kijanki,
MOSFETs ARE transistors. Field Effect, but still a transistor.
I have been in-fab for 30 years, the last making 'HexFets', a propriatary technology of my company...Bob Carver thought highly enough of them to use our devices some (all?) of his Sunfire amps.
This is not 'secret' info, but our 'discreet' (non IC devices) the entire bottom of the die is the drain. Since there is a conductive layer there(can't elaborate here) the RDON (resistance of the device ON) is lowered and when used as output devices results in a higher damping factor.....
Another device, not yet mentioned is the IGBT, Isolated Gate Bipolar Transistor. I have NO idea if there are audio applications here.
My company also has a line of class 'd' amps, one of which is out on my bench awaiting a power supply. One of these days!

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.