Metal Oxide Semiconductor Field Effect Transistors ( MOSFET )A metal-oxide-semiconductor field-effect transistor (MOSFET) has three terminals, source, gate, and drain. In a p-MOSFET (or n-MOSFET), both the source S and drain D are n-type (or p-type) and the substrate between them is p-type (or n-type). The gate and the p-type substrate is insulated by a thin layer of 
. Due to this insulation, there is no gate current to either the source or drain.
n In an n-channel MOSFET, the channel is made of n-type semiconductor, so the charges free to move along the channel are negatively charged (electrons).
n In a p-channel device the free charges which move from end-to-end are positively charged (holes).
Basic MOSFET (n-channel)
n The gate electrode is placed on top of a very thin insulating layer.
n There are a pair of small n-type regions just under the drain & source electrodes.
n If apply a +ve voltage to gate, will push away the ‘holes’ inside the p-type substrate and attracts the moveable electrons in the n-type regions under the source & drain electrodes.
n Increasing the +ve gate voltage pushes the p-type holes further away and enlarges the thickness of the created channel.
n As a result increases the amount of current which can go from source to drain — this is why this kind of transistor is called an enhancement mode device.
Cross-section and circuit symbol of an n-type MOSFET
Basic MOSFET (p-channel)
n PMOS behave in similar way, but they pass current when a -ve gate voltage creates an effective p-type channel layer under the insulator.
Types of MOSFET
