It refers to any transistor that uses an applied voltage (the field) as its mechanism to turn on or off. A basic transistor has three terminals. A FET's three terminals are usually called 'source', 'drain' and 'gate'. Applying a threshold voltage to the gate allows a current to flow from the source to the drain. a FET can be used as a switch simply by raising and lowering the gate voltage around its threshold value. FETs also operate as current amplifiers by increasing the gate voltage (above the threshold)
A MOSFET (metal oxide semiconductor field effect transistor) is the fundamental transistor behind most of todays electronics including computers. It has a metal contact for the gate and is separated from the bulk transistor (substrate) by an oxide layer, typically SiO2 (silicon dioxide). This makes sure no current flows through the gate.
Answered by:
Paul Speziale, B.S., Engineering Physics Grad Student, McMaster U.
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '