MOSFET

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor

device that is widely used for switching purposes and for the amplification of electronic signals in

electronic devices. A MOSFET is either a core or integrated circuit where it is designed and

fabricated in a single chip because the device is available in very small sizes. The introduction of

the MOSFET device has brought a change in the domain of switching in electronics. Let us go

with a detailed explanation of this concept.

A MOSFET is a four-terminal device having source(S), gate (G), drain (D) and body (B) terminals.

In general, The body of the MOSFET is in connection with the source terminal thus forming a

three-terminal device such as a field-effect transistor. MOSFET is generally considered as a

transistor and employed in both the analog and digital circuits. This is the basic introduction to

MOSFET. And the general structure of this device is as below :

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From the above MOSFET structure, the functionality of MOSFET depends on the electrical

variations happening in the channel width along with the flow of carriers (either holes or electrons).

The charge carriers enter into the channel through the source terminal and exit via the drain.

The width of the channel is controlled by the voltage on an electrode which is called the gate and

it is located in between the source and the drain. It is insulated from the channel near an extremely

thin layer of metal oxide. The MOS capacity that exists in the device is the crucial section where

the entire operation is across this.

A MOSFET can function in two ways

• Depletion Mode

• Enhancement Mode

Depletion Mode

When there is no voltage across the gate terminal, the channel shows its maximum

conductance. Whereas when the voltage across the gate terminal is either positive or negative, then

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the channel conductivity decreases. Please refer to this link to know more about Depletion Mode

MOSFET

Enhancement Mode

When there is no voltage across the gate terminal, then the device does not conduct. When there

is the maximum voltage across the gate terminal, then the device shows enhanced conductivity.

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Working Principle of MOSFET

The main principle of the MOSFET device is to be able to control the voltage and current flow

between the source and drain terminals. It works almost like a switch and the functionality of the

device is based on the MOS capacitor. The MOS capacitor is the main part of MOSFET.

The semiconductor surface at the below oxide layer which is located between the source and drain

terminal can be inverted from p-type to n-type by the application of either a positive or negative

gate voltages respectively. When we apply a repulsive force for the positive gate voltage, then the

holes present beneath the oxide layer are pushed downward with the substrate.

The depletion region populated by the bound negative charges which are associated with the

acceptor atoms. When electrons are reached, a channel is developed. The positive voltage also

attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied

between the drain and source, the current flows freely between the source and drain and the gate

voltage controls the electrons in the channel. Instead of the positive voltage, if we apply a negative

voltage, a hole channel will be formed under the oxide layer.

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P-Channel MOSFET

The P- channel MOSFET has a P- Channel region located in between the source and drain

terminals. It is a four-terminal device having the terminals as gate, drain, source, and body. The

drain and source are heavily doped p+ region and the body or substrate is of n-type. The flow of

current is in the direction of positively charged holes.

When we apply the negative voltage with repulsive force at the gate terminal, then the electrons

present under the oxide layer are pushed downwards into the substrate. The depletion region

populated by the bound positive charges which are associated with the donor atoms. The negative

gate voltage also attracts holes from the p+ source and drain region into the channel region. Please

refer to this link to know more about – P-Channel MOSFET.

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N- Channel MOSFET

The N-Channel MOSFET has an N- channel region located in between the source and drain

terminals. It is a four-terminal device having the terminals as gate, drain, source, body. In this type

of Field Effect Transistor, the drain and source are heavily doped n+ region and the substrate or

body are of P-type.

The current flow in this type of MOSFET happens because of negatively charged electrons. When

we apply the positive voltage with repulsive force at the gate terminal then the holes present under

the oxide layer are pushed downward into the substrate. The depletion region is populated by the

bound negative charges which are associated with the acceptor atoms.

Upon the reach of electrons, the channel is formed. The positive voltage also attracts electrons

from the n+ source and drain regions into the channel. Now, if a voltage is applied between the

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drain and source the current flows freely between the source and drain and the gate voltage controls

the electrons in the channel. Instead of positive voltage if we apply negative voltage then a hole

channel will be formed under the oxide layer. Please refer to this link to know more about – N-

Channel MOSFET.

MOSFET Characteristics Curves

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The minimum ON-state gate voltage required to ensure that the MOSFET remains “ON” when

carrying the selected drain current can be determined from the V-I transfer curves above.

When VIN is HIGH or equal to VDD, the MOSFET Q-point moves to point A along the load line.

The drain current ID increases to its maximum value due to a reduction in the channel

resistance. ID becomes a constant value independent of VDD, and is dependent only on VGS.

Therefore, the transistor behaves like a closed switch but the channel ON-resistance does not

reduce fully to zero due to its RDS(on) value, but gets very small.

Likewise, when VIN is LOW or reduced to zero, the MOSFET Q-point moves from point A to

point B along the load line. The channel resistance is very high so the transistor acts like an open

circuit and no current flows through the channel. So if the gate voltage of the MOSFET toggles

between two values, HIGH and LOW the MOSFET will behave as a “single-pole single-throw”

(SPST) solid state switch and this action is defined as:

MOSFET Regions of Operation

To the most general scenario, the operation of this device happens mainly in three regions and

those are as follows:

1. Cut-off Region
Here the operating conditions of the transistor are zero input gate voltage ( VIN ), zero drain

current ID and output voltage VDS = VDD. Therefore for an enhancement type MOSFET the

conductive channel is closed and the device is switched “OFF”. It is the region where the device

will be in the OFF condition and there zero amount of current flow through it. Here, the device

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functions as a basic switch and is so employed as when they are necessary to operate as electrical

switches.

Cut-off Characteristics

Then we can define the cut-off region or “OFF mode” when using an e-MOSFET as a switch as
being, gate voltage, VGS < VTH thus ID = 0. For a P-channel enhancement MOSFET, the Gate
potential must be more positive with respect to the Source.

2. Saturation Region
In the saturation or linear region, the transistor will be biased so that the maximum amount of gate
voltage is applied to the device which results in the channel resistance RDS(on being as small as
possible with maximum drain current flowing through the MOSFET switch. Therefore for the
enhancement type MOSFET the conductive channel is open and the device is switched “ON”.
In this region, the devices will have their drain to source current value as constant without

considering the enhancement in the voltage across the drain to source. This happens only once

when the voltage across the drain to source terminal increases more than the pinch-off voltage

value. In this scenario, the device functions as a closed switch where a saturated level of current

across the drain to source terminals flows. Due to this, the saturation region is selected when the

devices are supposed to perform switching.

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Saturation Characteristics

Then we can define the saturation region or “ON mode” when using an e-MOSFET as a switch as
gate-source voltage, VGS > VTH thus ID = Maximum. For a P-channel enhancement MOSFET, the
Gate potential must be more negative with respect to the Source.
By applying a suitable drive voltage to the gate of an FET, the resistance of the drain-source
channel, RDS(on) can be varied from an “OFF-resistance” of many hundreds of kΩ, effectively an
open circuit, to an “ON-resistance” of less than 1Ω, effectively acting as a short circuit.
When using the MOSFET as a switch we can drive the MOSFET to turn “ON” faster or slower,
or pass high or low currents. This ability to turn the power MOSFET “ON” and “OFF” allows the
device to be used as a very efficient switch with switching speeds much faster than standard bipolar
junction transistors.

3. Linear/Ohmic Region – It is the region where the current across the drain to source

terminal enhances with the increment in the voltage across the drain to source path. When

the MOSFET devices function in this linear region, they perform amplifier functionality.

Then we can summarise the switching characteristics of both the N-channel and P-channel type

MOSFET within the following table.

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Note that unlike the N-channel MOSFET whose gate terminal must be made more positive

(attracting electrons) than the source to allow current to flow through the channel, the conduction

through the P-channel MOSFET is due to the flow of holes. That is the gate terminal of a P-channel

MOSFET must be made more negative than the source and will only stop conducting (cut-off)

until the gate is more positive than the source.

So for the enhancement type power MOSFET to operate as an analogue switching device, it needs

to be switched between its “Cut-off Region” where: VGS = 0V (or VGS = -ve) and its “Saturation

Region” where: VGS(on) = +ve. The power dissipated in the MOSFET ( PD ) depends upon the

current flowing through the channel ID at saturation and also the “ON-resistance” of the channel

given as RDS(on).

Ideal Switch Characteristics

When a MOSFET is supposed to function as an ideal switch, it should hold the below properties

and those are

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 • In the ON condition, there has to be the current limitation that it carries

• In the OFF condition, blocking voltage levels should not hold any kind of limitations

• When the device functions in ON state, the voltage drop value should be null

• The resistance in OFF state should be infinite

• There should be no restrictions on the speed of operation

Practical Switch Characteristics

As the world is not just stuck to ideal applications, the functioning of MOSFET is even applicable

for practical purposes. In the practical scenario, the device should hold the below properties

• In the ON condition, the power managing abilities should be limited which means that

the flow of conduction current has to be restricted.

• In the OFF state, blocking voltage levels should not be limited

• Turning ON and OFF for finite times restricts the limiting speed of the device and even

limits the functional frequency

• In the ON condition of the MOSFET device, there will be minimal resistance values

where this results in the voltage drop in forwarding bias. Also, there exists finite OFF

state resistance that delivers reverse leakage current

• When the device is performing in practical characteristics, it loses power on ON and

OFF conditions. This happens even in the transition states too.

Example of MOSFET as a Switch

In the below circuit arrangement, an enhanced mode and N-channel MOSFET are being used to

switch a sample lamp with the conditions ON and OFF. The positive voltage at the gate terminal

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is applied to the base of the transistor and the lamp moves into ON condition and here VGS =+v or

at zero voltage level, the device turns to OFF condition where VGS=0.

If the resistive load of the lamp was to be replaced by an inductive load and connected to the relay

or diode which is protected to the load. In the above circuit, it is a very simple circuit for switching

a resistive load such as a lamp or LED. But when using MOSFET as a switch either with inductive

load or capacitive load, then protection is required for the MOSFET device.

If in the case when the MOSFET is not protected, it may lead to damage of the device. For the

MOSFET to operate as an analog switching device, it needs to be switched between its cutoff

region where VGS =0 and saturation region where VGS =+v.

How To Choose MOSFET as Switch?

There are few conditions to be observed while selecting the MOSFET as a switch and those are a

follows:

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 • Usage of polarity either P or N channel

• A maximum rating of operating voltage and current values

• Increased Rds ON which means that resistance at Drain to Source terminal when the

channel is completely open

• Enhanced operational frequency

• Packing kind is of To-220 and DPAck and many others.

What is MOSFET Switch Efficiency?

The main restriction at the time of operating MOSFET as a switching device is the enhanced drain

current value that the device can be capable of. It means that RDS in ON condition is the crucial

parameter which decides the switching capability of the MOSFET. It is represented as the ratio of

drain-source voltage to that of drain current. It has to be calculated only in the ON state of the

transistor.

Why MOSFET Switch is Used in Boost Converter?

In general, a boost converter needs a switching transistor for the operation of the device. So, as

switching transistor MOSFETs are used. These devices are used to know the current value and

voltage values. Also, considering the switching speed and cost, these are extensively employed.

Advantages

• It generates enhanced efficiency even when functioning at minimal voltage levels

• There is no presence of gate current this creates more input impedance which further

provides increased switching speed for the device

• These devices can function at minimal power levels and uses minimal current

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Disadvantages

• When these devices are functioned at overload voltage levels, it creates instability of

the device

• As because the devices have a thin oxide layer, this may create damage to the device

when stimulated by the electrostatic charges

Applications

• Amplifiers made of MOSFET are extremely employed in extensive frequency

applications

• The regulation for DC motors are provided by these devices

• As because these have enhanced switching speeds, it acts as perfect for the construction

of chopper amplifiers

• Functions as a passive component for various electronic elements.

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