explain MOSFET, Metal-Oxide-Semiconductor FET

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals. It is by far the most common field-effect transistor in both digital and analog circuits. The MOSFET is composed of a channel of n-type or p-type semiconductor material (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also commonly nMOSFET, pMOSFET).

This device is the reason that there are warning labels on almost all computer hardware - “static-sensitive device; handle with care”.  Memory in most cases turn out to be MOSFET switches, as is most of the circuitry on the CPU (incidentally the change in operating voltage for the CPU was a result of changing from BJTs to MOSFETs).
As with the JFET there is an additional layer (literally the wafer that it was grown on) that is normally not an external contact - it is internally connected to the source.

    How does it work?  There is no conduction between the source and drain normally (VGS = 0) because regardless of what voltage VDS you apply there is a reverse biased PN junction.  Even apply a voltage VGS does not appear from the structure to have an obvious effect since it is not even attached - there is a thin SiO2 insulating layer in between!  This gate oxide incidentally is very important - it is one of the current limitations on how fast computers run!

This charge must come from the substrate.  Since it is P-type there are not many electrons but those that are present are all sucked up to the gate oxide.  This creates a region that is very thin, but very rich in electrons, converting P-type to N-type locally.  This “channel” is enhanced by applying higher positive biases.
While there are many applications for MOSFETs (remember they are just like JFETs with the threshold voltage shifted higher) The dominant application is a switch.  Most of digital electronics is based on low power switches and most DC power supplies are based on high power switches.

For an enhancement-mode, n-channel MOSFET the three operational modes are:
Cut-off or Sub-threshold or Weak Inversion Mode
When V GS < Vth:
where Vth is the threshold voltage of the device.
According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source.

where ID0 = current at VGS = Vth and the slope factor n is given by
             n = 1 + CD / COX,
with CD = capacitance of the depletion layer and COX = capacitance of the oxide layer. In a long-channel device, there is no drain voltage dependence of the current once VDS > > VT, but as channel length is reduced drain-induced barrier lowering