For questions relating to the use of MOSFETs in 3D printers

A metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. These transistors are often used in power applications.

With respect to 3D printers, a MOSFET is most commonly used to provide additional power to a heated bed. Some controller boards are unable to provide sufficient current necessary to heat a heated bed, and so a MOSFET is used to provide a switching mechanism in order to supply the additional current.