What is a MOSFET? | Basics, Working Principle & Applications - Electrical C & V

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05 April 2020

What is a MOSFET? | Basics, Working Principle & Applications

What is MOSFET?

The MOSFET has a full form metal oxide semiconductor field effect transistor. It is a semiconductor device used in electronic devices for switching and amplifying electronic signals.


What is a MOSFET? | Basics, Working Principle & Applications
MOSFET

It is a four-terminal device whose terminals are source (S), gate (G), drain (D) and body (B). The body is usually attached to the source terminal, making it a device with three terminals, such as FET.
MOSFET can be used in both analog and digital circuits.

MOSFET works because the channel width is changed. A channel is where the charge carrier (electron or hole) flows from charge, enter the channel from carrier source and exit the drain. The width of the channel is controlled by applying voltage to the gate. The gate terminal is located between the source and drain. The gate terminal (metal oxide gate electrode) is separated from the channel by a very thin layer of silicon dioxide.


Construction of MOSFET

MOSFETs have the same structure as FETs. An oxide layer with which the gate is attached is attached to the substrate. This layer of oxide acts as a conductor. Therefore, another name of MOSFET is also IGFET (insulated gate field effect transistor).


What is a MOSFET? | Basics, Working Principle & Applications
Construction of MOSFET
                       
In the structure of the Mosfet a less doped substrate is transmitted to a more doped region. Depending on the type of substrate used, we can call MOSFET p type or n type MOSFET.


 MOSFET works in two ways -

1. Depletion mode
2. Enhancement mode

1. Depletion mode:- The channel shows maximum conductivity when there is no voltage at the gate. As soon as the voltage at the gate is positive or negative, the conductivity of the channel decreases. It is used to OFF the device.

2. Enhancement mode - MOSFET does not work when there is no voltage at the gate, then no current flows in it. As the voltage at the gate increases, the conductivity of the MOSFET increases. This is used to turn the device ON.


Operation of MOSFET 

The procedure of this depends on the MOS capacitor. The MOS capacitor is the main part of the MOSFET. Under the layer of oxide that is the surface of the semiconductor (between the source and drain), that surface can be converted from p type to n type, giving positive and negative voltage at the gate terminal.

When we apply a positive gate voltage, the holes under the oxide layer are pushed down into the substrate due to a repulsive force. The population of negative charge increases significantly in depletion area. This creates a channel with abundance of electrons.

Positive charges attract more electrons from the n + source and from the drain. Now if voltage is applied between drain and source, then current will flow between source and drain without interruption and gate will control electrons in voltage channel. If we apply negative voltage instead of positive voltage then a hole channel will be formed under the oxide layer.

P channel MOSFET

The p channel is the area of ​​the p channel between the source and drain in the MOSFET. It is a four terminal device with four terminals source, drain, gate and body.


What is a MOSFET? | Basics, Working Principle & Applications
P channel MOSFET
                                 

The drain and source are very doped p + fields and the body or substrate is of type n. The current flows through a positively charged hole.

When we apply a negative gate voltage, the electrons under the oxide layer are pushed downward into the substrate due to the repulsive force. The depletion zone is filled with positive charges that are connected to the donor atom. The negative gate voltage attracts more holes from the p + source and drain into the channel region. And thus the stream starts to flow between the source and the drain.

N channel MOSFET

It has n channel area between source and drain. This is a four terminal device. The drain and source are very doped n + type fields. And the substrate or body is p type.

What is a MOSFET? | Basics, Working Principle & Applications
N channel MOSFET
                                 
In this, current flows due to negatively charged electrons. When we apply positive gate voltage, the holes under the depletion layer are pushed downward into the substrate due to the repulsive force.

The depletion field is filled with negative charges that are associated with the acceptor atom. The positive voltage draws electrons from the n + type drain and source and into the channel. In this, current starts flowing between source and drain.


Applications of MOSFET

1) It is used in the switching of electronic devices.

2) They are used to amplify electronic signals.

3) It is used in digital circuits.

4) Mosfet can be used as a high frequency amplifier.

5) It can be used as a passive element (resistor, inductor and capacitor).

6) It can be used in electronic DC relay.

7) It can be used in switch mode power supply (SMPS).







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