The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) or MOS transistor is a type of transistor that consists of a metal layer, an oxide layer, and a semiconductor layer.  The semiconductor layer is usually in the form of single-crystal silicon substrate doped precisely to perform transistor action.  The oxide is usually in the form of a silicon dioxide layer that insulates the semiconductor layer from the metal layer.  The metal layer is used as contact for providing voltage inputs to the MOS transistor.

The MOS transistor consists of three terminals:  a gate, a source, and a drain.  These are equivalent to the base, emitter, and collector of a bipolar transistor.  The metal layer of the MOS transistor serves as the gate, while the source and drain are fabricated on the silicon substrate.

Like a bipolar transistor, the current flowing through a MOS transistor is controlled by the input at its gate.  However, unlike a bipolar transistor which is controlled by the amount of current into its base, a MOS transistor is controlled by the voltage level at its gate.

The source and drain of a MOS transistor are created on the silicon substrate in such a way that they are 'sandwiching' the gate.  The source and drain are doped to be of the same material type, which should be different from the doping received by the substrate.  A MOS transistor is referred to as a P-channel MOSFET, or PMOS, if the source and drain are p-type, and the substrate is n-type.   It is an N-channel MOSFET, or NMOS, if the source and drain are n-type, and the substrate is p-type. 

The area under the gate is known as the channel.  The conductivity of the channel may be controlled through the voltage level applied to the gate.   For instance, in an NMOS, the major carrier is the electron, so the channel becomes more conductive by applying a positive voltage at the gate, which tends to attract more electrons from the substrate into the channel.  The layer formed by these attracted electrons is known as the 'inversion layer', since electrons are the minority carriers of the p-substrate.

If the source of the NMOS is more negative than the drain while a sufficiently positive voltage is applied to the gate, current would pass through the transistor.  Removing the positive voltage at the gate would significantly decrease the conductivity of the channel, constricting the flow of electrons.  A MOS transistor operating in this manner is known as an enhancement-mode MOS transistor, because it is normally open and conducts only when the channel is 'enhanced.'  On the other hand, a normally conducting transistor is known as a depletion-mode transistor, since its conduction is controlled by 'depleting' the normally-present channel.