Nearly all amplifiers utilize this in some form or fashion, ALL switching amps use MOSFETS. MOSFETs stand for Metal Oxide Semiconductor Field Effect Transistor, basically, it's a J channel transistor, the difference is that the gate is external and isolated from the channel. Mosfets are ideal switching devices, and A/B, B, C, D, etc. amps switch to save power and heat because it uses seperate transistors for each wave. Basically, one transistor is used for the positive wave, and one is used for the negative. Mosfets are what turn these transistors on or off, switching between the two, and do so by the bias that they are wired. Other transistors are J channel (not used in car audio, at least not to my knowledge or not often) and bipolar transistors (rarely used, but better sonic performance than a mosfet b/c it's more linear, with the cost of being less reliable). Vacuum tube or "valve" amps are another type of amp, and are rarely used in car audio.
If I remember correctly Mosfets just started to become popular when I really started getting into car audio like around '86 I guess. "Mosfet" was a huge buzzword back then. If you had a mosfet amp you were stylin. There were a lot of amps out there that sounded decent but weren't utilizing them.
you wish You still need an alternator, it's not that easy. Nearly all car audio amps use a MOSFET power supply b/c the voltage is low (12-14.4V typically). This needs to be stepped up to create meaningful power. When MOSFETs are used in a power supply, it means a switching power supply. How it works is the mosfets switch at high rates (20,000-50,000 pulses per second) creating pulsating DC, this creates a magnetic field (similar to AC). Then, that is sent to a transformer, and the transformer increases the voltage. The secondary windings of the transformer are in AC, and they are then smoothed via a rectifier (diodes) to power the amp. Transistors used for the audio signal are different. Transistors are designed flow one way(like a diode), the difference from a diode is that they add another junction, they have either two positive or two negative(depending on transistor type) and one of the opposing junctions(two positive and one negative, or two negative and one positive). MOSFETs are also typically used as the transistors for the input signal as well. Power is applied to the transistors, and this power flows electrons through the transistor and is controlled by the input signal (RCA signal, or music). When the music has a positive wave, that wave will go through the transistor and electrons will be "pulled" from the power supply, resulting in the same signal, just at a higher power level. Think of it like the venturi effect. This is amplification. The reason that MOSFETS are so good for it is that they work well off of low signals, meaning they are very fast to respond to a signal because their internal (gate) voltage is low, meaning you need less power to overcome the internal voltage of the transistor, and thus, power will be amplified more quickly.
I'm probably not saying this correctly, but I was heavy into car-fi when they became the "buzz-word" and seem to remember that pretty much everything that "amplifies" uses a FET (Field Effect Transistor) and what made these special was the fact that they were a MOS (Metal Oxide Semiconductor) version of the standard FET. I thought that every amp since the dawn of time (not really) used FET's and the MOS portion was a technicalogical breakthrough in the mid 80s and originally only available on high end gear as Fishy said. I could be wrong though.
"Mosfets are what turn these transistors on or off, switching between the two, and do so by the bias that they are wired" When I said that above, I meant to say MOSFETS are what turn on or off and switch according to the bias they're wired for. MOSFETs don't turn other transistors on or off. Just clearing up if I confused anyone.
They started appearing earlier than that, but were rare. Really, a MOSFET is worse than a bipolar transistor for audio output, but they're more rugged than bipolars, and in a car that's a good thing,. Their audio performance isn't very linear and they have to have a lot of feedback to get low distortion. They also can't be perfectly matched, making crossover distortion worse. But, they switch fast, meaning they work well w/ switching power supplies and A/B, B, C, D amps, etc. because they can control power with a low gate voltage. FETs were used a while back, but today MOSFETS are the primary transistor of choice in car amplifiers. A select few high end companies use bipolars b/c of the better sonic performance, but bipolars are very rare in car audio. About as rare as car tube amps.
They may be used as a switching power supply. Could be used as the transistors too, can't remember. A lot of A/B amps are "deemed" Class A from companies b/c of a high bias voltage, meaning the transistors have little to no crossover distortion b/c they're always on the verge of turning on, but literally, they're still Class A. Not sure if that Soundstream is one of companies that does that, though. Pretty much all car audio amps use either a MOSFET power supply, some high end amps use pulse width modulation.