Introduction

A Field Effect Transistor (FET) in which the Gate input is electrically insulated from the main current carrying channel is called Metal Oxide Semiconductor Field Effect Transistor (MOSFET). This type of FET is also sometimes called Insulated Gate FET (IGFET).

The classification of MOSFET devices is based on the type of the conducting channel being used.

If the conducting Channel is of N - type, such a device is called as N Channel MOSFET or in short NMOS. 

SImilarly If the conducting Channel is of P - type, such a device is called as P Channel MOSFET or in short PMOS. 

The MOSFET is a Voltage Controlled Field Effect Transistor similar to that of a FET and the difference between the two devices is that in the MOSFET device the Gate electrode is made up of Metal Oxide. This Metal Oxide Gate Electrode is electrically insulated from the main semiconductor channel - N Channel or P Channel by a very thin layer of insulating material which is Silicon Dioxide.

As the insulating material Silicon Dioxide isolates the controlling Gate, the input resistance of the MOSFET is extremely high (in M Ω region). As such the input resistance of the MOSFET device is far higher than that of the FET device and is almost infinite.

Note: 

The effect of the very large input resistance of the MOSFET is that it accumulates a large amount of static charge. The presence of the large static charge in MOSFET easily damages it and thus requires careful handling or extra protection.