What is MOSFET?

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Metal-Oxide semiconductor field effect transistor (MOSFET) has an oxide layer between gate & channel to increase the input impedance & reduce the overall circuit current.
The extraordinarily high input impedance means that these circuits are able to consume very low power levels.

Principal of MOSFET - By applying a transverse electric field across an insulator (oxide layer), deposited on the semiconducting material, the thickness & hence the resistance of a conducting channel of a semiconducting material can be controlled.

Classification of MOSFET - MOSFET is classified in two types.
P-ch = P channel           N-ch = N channel 

Construction of MOSFET - MOSFET is a four terminal device having source(S), gate(G), Drain(D), & body substrate (B) terminals. An oxide layer is deposited on the substrate to which the gate terminal is connected. The oxide layer act as a insulator.
 
Depletion MOSFET - In a depletion MOSFET, the controlliy electric field reduces the number of majority carrier available for Conduction.
The depletion MOSFET can be operated in both depletion mode & enhencement mode. For this reason, it is also called dual mode MOSFET.
Types of depletion MOSFET - There are two types.
(1) N-Channel depletion MOSFET
(2) P-Channel depletion MOSFET              N- channel depletion MOSFET - The N-channel MOSFET is a piece of n- type material with a P-type region called substrate. The free electrons flowing from source to drain (when drain is positive with respect to source) most pass through the narrow channel between gate & P-type region. Therefore,the drain current flow through the channel drain to source.
 If the gate voltage is made negative, positive charge consisting of holes is induced in the channel through silver oxide layer. The shape of depletion region depends on gate to source voltage & drain to source voltage.When drain to source voltage increased, drain current increases & it becomes parctically constant at a certain value of drain to source voltage, called the pinch off voltage.
The depletion MOSFET may also be operated in an enhancement mode. It is necessary to apply a positive gate voltage so that negative charges are induced into the N-type channel. Hence the conductivity of the channel increases & drain current increases.
The curve of drain current versus gate to source voltage for constant drain to source voltage is called transfer characteristics of MOSFET.
Enhancement MOSFET - In the enhancement MOSFET, application of electric field causes an increase in the majority carrier density in the conducting regions of the transistor.
Types of enhancement MOSFET - There are two types.
(1) N-Channel enhancement MOSFET
(2) P-Channel enhancement MOSFET
N-Channel enhancement MOSFET - Two highly doped N-regions are diffused in a lightly doped P-type substrate.One N-region is called the source (S) & other one is called the drain (D).They are separated by 1 mil(10–³ inch).A thin insulating layer of silicon oxide is grown over the surface of the structure & a thin layer of metal aluminium is formed over the layer of silicon oxide. This metal layer covers the entire channel region & it from the gate (G).
Enhancement MOSFET is called the insulated gate FET because is the insulating layer of silicon oxide. This layer gives an extremely high input impedance for the MOSFET.

If the positive voltage is applied at the gate, the positive charge on gate induces an equal negative charge on the substrate side between the source & drain regions. This ,an electric field is produced between the source & drain regions.As the positive voltage on gate increases,the induced negative charge in the semiconductor increases. Hence, the conductivity increases & current flows from source to drain through the induced channel.

Comparison between BJT, FET & MOSFET 

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