MOSFET

09-01-09: Kijanki
There is no perfect device. Mosfets output stages are:

- more robust than transistors
- have higher bandwidth
- are more tolerant of difficult loads

but are:

- less efficient since have lower voltage swing from the same supply voltage
- are more nonlinear than transistors around cutoff region
- require more gain in preceding stages to eliminate crossover distortions
- limited ability to apply local feedback (low gain)

Kijanki, some of the material you have written doesn't make sense to me & also appears to be contradictory: in the advantages you wrote that MOSFET have higher bandwidth than BJTs. In the disadv you wrote that they have lower voltage swing from the same supply voltage. This is contradictory to me.
Lower voltage swing from the same voltage supply compared to the BJT implies lower gain. Bandwidth & gain (Gm) are directly related. Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth. This seems to be what I observe too.

I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs. Ditto re. the ability of MOSFETs to handle difficult loads any better than BJTs.

MOSFETs are a square law device hence the distortions are more along the tube-like behaviour. That's why one often reads of MOSFETs being tubey sounding. In comparison the BJT is an exponential device & the distortions are odd-order harmonic based. If the implementation is not correct/good enough this can lead to very discordant sound very quickly.

I agree w/ most of the other members that MOSFETs should sound pretty good if implemented "correctly" (another audio fuzzy term!). I believe that even better than the MOSFET is its cousin the JFET in the output stage - MOSFET electrical characteristics but BJT-like sound. Preferred by the discriminating amp builders. The sad part is that no one is making large quantities of power JFETs like they are power MOSFETs & BJTs. Thus amps seldom use JFETs.
Now, First Watt *seems* to be using JFETs since they output only a few watts.

I think I read that the C-J 350 used JFETs, but not certain.

"Lower voltage swing from the same voltage supply compared to the BJT implies lower gain" - No it has nothing to do with gain but with characteristic.

"Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth". - No it doesn't. Even between bipolar transistors current gain has nothing to do with max bandwidth. It is just matter of design. Mosfets are generally much faster.

"I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs" - sure they are more robust since Mosfets don't suffer secondary breakdown (hot spots of current on the wafer with higher voltages resulting in thermal avalanche - very nasty complication) and are thermally stable (BJT's Vbe drops with temperature increasing collector current hence increasing temperature - thermal runaway). Mosfet drain-source resistance has a positive temperature coefficient, so they tend to be self protective.

As for tubey sound - Mosfets have softer clipping than BJts but are more nonlinear and require much higher gain before feedback to correct it - in result behave pretty much the same as BJts. Higher gain results in design more prone to TIM and therefore higher order odd harmonics.

Just think about it - if one type of devices would have clear advantage nobody would use other devices. Some manufacturers (Krell, Levinson etc) use bipolar designs.

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!

Magfan - Isn't HexFet a vertical Mosfet designed for switching (low resistance)?

I'm just saying that there is no perfect device. Mosfets have some advantages as well as BJts and tubes. I use class D Rowland amp where Mosfets are a must.