A MOSFET is a metal-oxide-semiconductor FET. This transistor is a type of FET that uses an induced electrical field to control current through the device. 

The substrate is usually connected to the most negatively biased part of the MOSFET, usually the source lead. In three-terminal MOSFETs, the substrate is internally connected to the source. In four-terminal MOSFETs, the substrate is connected to an external substrate terminal.

A MOSFET can be operated in its saturation region for amplification applications. It is commonly operated in its cut-off and linear regions for switching applications, such as those in digital logic or in switched-mode power supplies.