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MOSFET: Definition, Symbol, Working, Characteristics, Types & Applications

elizabethjones4256elizabethjones4256 wrote 09/13/2021 at 11:30 • 6 min read • Like

MOSFET:

    MOSFET is known as a metal oxide field effective transistor, they are an advanced form of FET as FET have some limitations in their operations, like impedance and frequency limitations, so MOSFETS are used in those circuits where high affectivity is required.

Construction:

Like other transistors, MOSFETs are also made up of semiconductors known as silicon. Semiconductor materials are partially conductive so their properties were increased by a process known as doping. Doping is a technique through which impurities were introduced in the Semiconductor devices to increase their functionality. 

Like phosphorus is added as an impurity in the silicon to increase its conductivity, as phosphorous has an excess amount of electrons present in it, which increases the number of free electrons in the silicon, which results in increasing its conductivity, this opening is known as P-type doping. Similarly when boron is added to the silicon as an impurity the number of holes increased. This doping is known as N-type doping. MOSFET is also a combination of these N-type and P-type Doing. 

MOSFET has four basic body parts known as Drain, Gate, Body, and source, and they were attached in the circuit with three terminals that are Gate, Body, and Source. MOSFET is made by injecting impurities in silicon and make Gate and Source as P and N-type semiconductor that is clearly shown in the figure.

 The construction of the gate terminal is such that a dielectric is placed above the silicon semiconductor with a metal-metal plate above the dielectric. Thus it makes a structure just like a capacitor. As capacitor contains a dielectric between two plats to store charge here Gate terminal acts similarly by using this combination. A clear glimpse of the structural view of MOSFET is shown in the figure.

Working:

    The main working Principle of the MOSFET is to control the flow of electricity or voltage between the drain and the source. The semiconductor is doped with either Type or N-type. When the voltage is applied between drain and gate, by connecting Positive terminal with the metallic plate, due to the presence of dielectric Positive charge is start storing in it.

Which further attaches the electron present in the semiconductor, and start pushing the holes downwards in the substrate. These electrons form a thick layer with the dielectric that helps in flowing electrons from source to drain. The thickness of the layer is controlled by the voltage applied.

Conversely when the negative potential is applied then the Holes were attracted towards the plate and electrons are pushed away towards the substrate, forming a depletion region in the substrate. Based on the operation and functionality, the MOSFET working is divided into two different modes known as.

Each mode is further divided into two different types known as 

Characteristics:

    Drain characteristics:

        Drain characteristics of MOSFET for drain current and voltage between drain and source are shown in the graph. As we change the source gate violate we see an increase in drain current initially then become constant with the drain-source voltage increment.

P channel:

    As for the P channel, the substrate is N-type while drain and source are heavily doped with P-type, and when the negative voltage is applied the holes are attracted towards the gate and electrons start moving towards the Base or body terminal. The enhancement and depletion mode is depicted in the figures. For these conditions to fill it is mandatory to supply negative voltage to the gate terminal.

   

In depletion mode, the polarity of the battery is reversed.

N-channel:

The circuit depicts that in type channel the substrate is drain and source have N-type doping and when positive voltage terminal is applied to the gate-source, the negative electrons form a layer with gate terminal and holes were attracted toward the body terminal as it is connected with the negative terminal of the battery. A picture of enhancement mode can be observed by the figure.

Cutt off region:

    In the MOSFET operational condition, Cutt off region is known as that region in which no current is passing throughout the circuit, and MOSFET is behaving just like a traditional switch for cutting off the source current.

Saturation Region:

    When the device reached its pinch-off state, the current passing through the MOSFET from source to drain is in its maximum state. Besides that, the voltage is increasing but the current remains constant because it is the maximum potential that can be delivered between source and drain.

Linear ohmic Property:

MOSFET is known as linear or ohmic when the current from drain to source increases as the Voltage applied to the gate terminal increases. As voltage bandwidth increases and conductivity of electrons were increased. Similarly when we decrease the voltage the current flow decreases.

Applications OF MOSPHET in Embedded systems:

MOSFET as a switch:

    Nowadays MOSPHET is highly useful as a switching device, because of the advanced control features, they are mostly used in circuits like HID lamps. To control the effectiveness of street lights by using MOSFET as a switch which can reduce energy cost and improve the brightness of light as per requirement.

 

Let’s take a simple example of a LED control system that uses MOSFET as a switch with a raspberry pie microprocessor to operate light signals like their brightness adjustment, giving on and off signal, and much more.

Some other applications of MOSPHETS are 

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