The name MOSFET describes the structure and the function of the transistor. MOS refers to the fact that a MOSFET is built by layering metal (the "gate") on oxide (an insulator which prevents the flow of electricity) on semiconductor (the "source" and "drain"). FET describes the action of the gate on the semiconductor. An electric signal is sent to the gate, which creates an electric field that alters the connection between the "source" and "drain".