What's a MOSFET?
MOSFET stands for M
Both tubes and transistors amplify signals by passing current from one side of the device to the other, sculpting it along the way to the same shape as a much weaker input signal. It's like a movie or slide projector - a source of energy (the bulb) is shaped by the film, and projected on the screen, where we see a much bigger version of the image (even though the actual light we see comes from the bulb, not the film).
There are basically three kinds of transistor that are used to amplify audio: the most common is a bipolar transistor. It is a sandwich of three layers of silicon, with the outer ones negatively charged and the middle one positively charged (NPN). Or, the other way around (PNP). A small signal on the middle layer controls a much bigger current passing between the two outer layers.
A later development was the Field-Effect Transistor (FET). Here the current doesn't have to pass through the middle layer of the sandwich, it passes near it, and is controlled by the field effect exerted on it. This was more efficient in a number of ways. It also happens to clip more softly than a bi-polar transistor.
The third type is an FET where the element doing the controlling doesn't even contact the channel carrying the large current. It's insulated from it by a thin layer of silicon dioxide - a kind of glass. This is the MOSFET. In certain configurations, it clips very softly.
There are some products in which MOSFETs are used as diodes in a "Mosfet Edition" of a tubescreamer-type diode clipper. In this application, the MOSFET is "off" at all times, and it is the PN boundary between the body and one of the terminals that is acting as a diode. This diode may have some favorable characteristics, but this has nothing to do with the soft-clipping transfer function that MOSFET gain stages are known for.
The clipping characteristics of individual vacuum tube or solid-state semiconductors are by no means the whole story in the behavior of a circuit. You've probably noticed by now that a circuit can have tubes in it and still sound really bad. And the sound that formed the original criterion for what's desireable in overdrive, the sound of a cranked non-master-volume tube amp, has got to do with a lot things besides the tubes. There's transformers, speakers and the interaction of these with the tubes, to say nothing of the acoustic and psycho-acoustic byproducts of playing loud. Anyone interested in getting a repeatable sound that isn't dependant on playing at a certain sound pressure level would be better off discarding the dogma surrounding tubes and transistors, and employing the only devices that can be trusted -- the ears.