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The MOSFETs or Metal Oxide-Semiconductor (MOS) are field effect devices that use the electric field to create a conduction channel.
MOSFETS are more important than JFETs because almost all Integrated Circuits (IC) are built with the MOS technology.
There are two kinds of MOS transistors:
N channel MOSFET or NMOS and
P channel MOSFET or PMOS.
At the same time they can be enhancement transistors or depletion transistors. In the present days the last ones are not used.
In these tutorials we will describe only the enhancement MOS transistor.
The following picture shows the different symbols used to describe the MOS transistor.
The next picture shows the N channel MOSFET physical structure with its four terminals: Gate, Drain, Source and Substrate. Normally the Source and the substrate are connected together.
 The Gate with W and L dimensions is separated from the substrate by a dielectric (SiO2), creating a similar structure of the capacitor plates.
If a positive voltage is applied to the gate, negative charges are induced (inversion layer) on the substrate surface and they create a conduction path between the Drain and Source terminals.
The minimum voltage needed to create the inversion layer is called threshold voltage (VT). This is a characteristic feature of the transistor.
If VGS < VT, the drain-source current is zero. Typical values for this voltage are between 0.5 and 3 volts.
<<FET Saturation and Breakdown regions
MOSFET: Cutoff and Lineal regions>>
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