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Construction – Characteristics – MOSFET as a Switch – CMOS basic concept.
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals.
MOSFETs have two regions, called the source and drain which are heavily doped. These are embedded in a substrate, which is doped the other way. The gap between the source and drain regions, which spans the substate, is where the current will eventually flow. A layer of insulating oxide is placed over this gap (the channel), and on top of that, a gate contact, usually made of polysilicon.
The way a MOSFET works is to modify a thin layer of this gap, or channel, using an electric field that propagates through the insulation. This modification could be either increasing the current carrying capacities of the channel, or reducing it. Thus, we have two kinds of devices - enhancement mode MOSFETs and depletion mode MOSFETs. Depending on what kind of silicon is in the channel, a MOSFET can be p-channel or n-channel. A p-channel MOSFET has p-type silicon in the channel when 'on.' This will be explained more clearly soon.
Like the junction FET there are two categories of MOSFET, namely p-channel and n-channel types. The principle of operation is similar, but has some significant differences. Whilst the channel area, and hence its conductivity of a JFET is controlled by the depletion layer of a reverse biased diode, for the MOSFET, the gate is placed on an insulating oxide layer. Voltages applied to the gate control the conductivity of the channel as a result of the electric field induced capacitively across the insulating dielectric layer. There are also two further categories of MOSFET: enhancement and depletion types. As the name suggests the depletion mode MOSFET acts by depleting or removing the current carriers from the channel, whereas the enhancement type increases the number of carriers according to the gate voltage.