In general terms, an amplifier is a device that takes a signal and increases its amplitude. It is part of what is known as an audio playback chain, which typically includes an audio source, such as a CD or record player, and other devices, like pre-amps and equalizers, and ends in a speaker. A MOSFET amplifier is a subcategory of amplifiers that employs metal–oxide–semiconductor field-effect transistor (MOSFET) technology to process digital signals with relatively low power consumption. Once limited to only high-end products, by the early 21st century, MOSFET circuits are the design of choice in more than 99% of microchips manufactured worldwide, including those used in amplifiers.
The idea of using metal-oxide as an insulator in a transistor dates back to the mid-1920s, when the general concept was first proposed and patented in the U.S. by Austro-Hungarian scientist Julius Edgar Lilienfeld. His idea was that metal-oxide layers could control the voltages pushed through a transistor with less of the performance hindrances that plagued existing field-effect transistor (FET) designs. True MOSFET, as it would be recognized today, was developed in the 1960s by Dawon Kahng, who demonstrated the first successful example of an insulated-gate field-effect transistor with gates comprised of metal, oxide, and silicon.
Following its transition to the commercial market in the mid-1960s, MOSFET transistors soon found a natural home in amplifier circuits. They replaced the bulkier and more expensive to manufacture vacuum tubes, and surpassed other kinds of insufficient transistors. Also, a stereo or home theater with a MOSFET amplifier generated less heat and used less power than competing technologies, such as bipolar junction transistors (BJTs).
MOSFET transistors came into their own in the digital age. Ideally suited to complementing integrated circuits, the increased demand for them led to lower production costs and widespread adoption into mid- and low-range electronics. Amplifiers that used them found a particular niche in mobile audio, which expanded greatly from the 1980s onward. This industry, by its nature, places significant emphasis on parts generating as little heat and being as small as possible. By enabling the use of relatively powerful amplifiers in dashboard head units, carmakers and aftermarket electronics manufacturers could appeal to audiophiles and justifiably market their products as reasonable mobile alternatives to traditional home theaters.
In the decades since its introduction, the ubiquity of MOSFET technology in modern circuitry has generally nullified the advantages of using it in marketing copy. Interestingly, it nevertheless remains prevalent in home and mobile audio advertising. Car radios and home theaters still often purport the benefits of MOSFET, despite the fact that it would actually be quite difficult to find an amp on the market built with any other kind of transistors. It is only a slight exaggeration to say that a car stereo advertised on the strength of its MOSFET amplifier is the same as advertising a book on the basis of it being made out of paper.