MOSFET: Construction, Principles and Types

Content Curator

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a type of Field effect transistor (FET) used for amplifying or switching electronic signals.

It is most commonly fabricated by the controlled oxidation of silicon.
The MOSFETs have an insulated gate, and the conductivity of the device is determined by the voltage of the insulated gate.
Its main advantage over Bipolar junction transistors (BJTs) is that it require almost no input current to control the load current.
They were invented to solve the problems such as slow operation, moderate input impedance, and high drain resistance.
There are billions of MOSFET transistors in a microprocessor, making them the most common transistor in digital circuits.

What is a MOSFET?

[Click Here for Sample Questions]

Metal oxide semiconductor field effect transistors (MOSFETs) are electrical devices that switch or amplify voltages in circuits.

They use an insulated gate to control conductivity with voltage, enabling amplification and switching of electronic signals.
It is a voltage-controlled device.
A metal-insulator-semiconductor field-effect transistor (MISFET) and insulated-gate field-effect transistor (IGFET) are termed almost synonymous with MOSFET.

Generally, MOSFET is a four-terminal device. MOSFET terminals are labeled as follows:

Source
Gate
Drain
Body

The body is connected to the source terminal, reducing the terminals to three.

MOSFET

Read more:

Important Topics
Junction Transistor	Difference between npn and pnp transistor	Types of Transistors
Uses of Transistor	Transistor as switch amplifier	P-n junction

Construction of MOSFET

[Click Here for Previous Year Questions]

A MOSFET can be constructed as follows:

The body of a MOSFET is typically made from a p-type semiconductor.
The source and drain terminals are formed from heavily doped n-type regions on either side of the body.
These doped regions are often denoted by n⁺ in diagrams to indicate their high doping concentration.
A layer of silicon dioxide is deposited on the silicon substrate to provide electrical isolation.
A thin metallic layer is deposited on top of the silicon dioxide, forming a capacitor-like structure.
This metallic layer is then patterned and etched away to define the gate terminal.
A voltage source is then connected between these two n-type areas to construct a DC circuit.

MOSFET

MOSFET circuit diagram

Working Principle of MOSFET

[Click Here for Sample Questions]

The main working principle of a MOSFET device is to be able to control the current flow and voltage between the drain and source terminals. It works almost like a switch.

The operation of a MOSFET is dependent upon the MOS capacitor. The MOS capacitor is the heart of the MOSFET.
The semiconductor surface can be inverted from n-type to p-type by applying either negative or positive gate voltages respectively.
Positive gate voltage pushes holes deeper into the substrate, allowing electrons to flow.
The depletion region is populated by bound negative charges associated with the acceptor atoms.
A channel is developed when electrons are reached.
Electrons are also attracted to the positive voltage from the n⁺ source and drained into the channel by that voltage.
As a result, if a voltage is applied between the drain and source, current flows freely between them, and the gate voltage controls electron flow.
If we apply a negative voltage, a hole channel will be formed under the oxide layer, instead of the positive voltage.

Types of MOSFET

[Click Here for Previous Year Questions]

There are two types of MOSFETs

Enhancement
Depletion.

Each class is available in either n-channel or p-channel configurations, resulting in a total of four MOSFET categories.

The flowchart below shows how MOSFETs are classified based on their structure and materials.

Types of MOSFET

Depletion Mode

The Depletion mode P-N channel has the highest conductivity when there is no voltage at the gate terminal. The channel conductivity diminishes when the voltage across the gate terminal is either positive or negative.

Enhancement Mode

The circuit does not work if there is no voltage across the gate terminal.

When the maximum voltage is applied across the gate terminal, the device's conductivity improves.
The abbreviation NMOS is used to refer to N-channel MOSFETs and the P-channel MOSFETs are abbreviated as PMOS and the circuit looks as follows:

Enhancement and Depletion mode of MOSFET

MOSFET Operating Regions

[Click Here for Sample Questions]

Three functioning areas can be seen in a MOSFET:

Cut-Off Region

The cut-off region is a region where no conduction occurs, and so the MOSFET is turned off. MOSFET functions like an open switch in this situation.

Ohmic Region

The ohmic region is defined as a region where the current (I_DS) rises as the value of V_DS rises. MOSFETs are used as amplifiers when they are designed to function in this range.

Saturation Region

The I_DS current of MOSFET remains constant despite an increase in V_DS in the saturation area, which occurs when V_DS reaches the pinch-off voltage V_P. In this case, the device will operate as a closed switch, allowing a saturated value of I_DS to pass through it. As a result, whenever MOSFETs are required to perform switching operations, this operating area is picked.

MOSFET vs Bipolar Junction Transistor(BJT)

[Click Here for Previous Year Questions]

The main differences between MOSFETs and BJTs are-

MOSFET	BJT
MOSFET stands for Metal–oxide–semiconductor field-effect transistor.	BJT stands for Bipolar Junction Transistor
MOSFET is a four-terminal semiconductor device.	BJT is a three-terminal semiconductor device.
MOSFET is a unipolar device.	BJT is a bipolar device.
Either electrons or holes act as charge carriers depending on the type of channel between the source and drain in a MOSFET.	Both electrons and holes act as charge carriers in a BJT
It has a positive temperature coefficient.	It has a negative temperature coefficient.
There are two types of MOSFET and they are named: P-type or N-type.	PNP and NPN are the two different forms of BJTs.
It is a voltage-controlled device.	A bipolar junction transistor (BJT) is a current-controlled device.
The input resistance of MOSFET is high.	BJTs have a low input resistance.

Applications of MOSFET

[Click Here for Sample Questions]

MOSFET is used in many electronic projects such as light intensity control, motor control, and max generator applications. More applications of MOSFETs are-

Its amplifiers are widely used in radiofrequency applications.
The MOSFET is a passive circuit component.
DC motors can be regulated using power MOSFETs.
The chopper circuit is designed using MOSFETs.
It is used as a switch to operate as an analog switching device.
The auto intensity control of street lights is done using MOSFETs.

Advantages of MOSFET

[Click Here for Previous Year Questions]

MOSFET is a high voltage controlling device that provides key features for circuit designers in terms of the overall performance of the circuit. Some more advantages of MOSFET are-

At lower voltages, MOSFETs have a higher efficiency.
Due to the lack of gate current, the input impedance is high, resulting in a high switching speed.
MOSFET has low cost and fast transient response.
The oxide layer between the gate and channel prevents DC flow power consumption
MOSFET’s low power consumption allows more components per chip surface area.

Disadvantages of MOSFET

[Click Here for Sample Questions]

The disadvantages of MOSFETs are-

MOSFETs are susceptible to electrostatic charge damage because of their thin oxide layer.
They become unreliable when they are overloaded.
They require thin oxide which increases the chances of leakage problems.
MOSFET is a Unipolar voltage device.
Increased process fabrication steps- complex circuit design.

Things to Remember

MOSFETs, or Metal Oxide Silicon Field Effect Transistors, are four-terminal current-controlled devices used for switching and amplification.
MOSFETs are bidirectional transistors.
There are two types of MOSFETs: enhancement and depletion.
Each class is available in either n-channel or p-channel configurations, resulting in a total of four MOSFET kinds.
The cutoff region, the ohmic region, and the saturation region are the three operational regions.
A voltage-controlled device is a MOSFET, whereas a current-controlled device is a BJT.

Read more:

Important Topics
Bridge Rectifier	Full Wave Rectifier	Half Wave Rectifier
Electrostatic Conductor	Power Transformers	Parallel Current Carrying Conductor
Ncert Solutions of Semiconductor Electronics	Semiconductor Electronics Important Questions	Semiconductor Electronics MCQs

Previous Year Questions

Which logic gate does the following truth table represent?… [JKCET 2012]
If an emitter current is changed by 4mA, the collector current… [JEE Main 2021]
The impurity atoms with which pure silicon should be doped to make… [JEE Advanced 1988]
The half-life of the radioactive radon is 3.8 days. The time… [JEE Advanced 1981]
Pure silicon is a/an… [JIPMER 2018]
In intrinsic semiconductors at room temperature number of electrons… [MET 2012]
The device that can act as a complete electronic circuit is… [NEET 2010]
Zener diode is used for… [NEET 2005]
When the n-p-n transistor is used as an amplifier, then… [NEET 1996]
The increase in the width of the depletion region in a p-n junction diode… [NEET 2020]
A p-n junction diode can be used as… [NEET 1999]
In a p-n junction photocell, the value of the photo-electromotive force… [NEET 2005]
In which of the following statements, the obtained impure… [NEET 2013]
In a common emitter transistor amplifier the audio signal voltage across the… [NEET 2017]
The given electrical network is equivalent to… [NEET 2017]
p-n junction is said to be forward-biased, when… [NEET 1988]
At absolute zero, Si acts as… [NEET 1988]

Sample Questions

Ques. Describe the operation of a MOSFET. (3 Marks)

Ans. A MOSFET can function in two different ways.

The first one is called depletion mode. The channel has its maximal conductivity when there is no voltage on the gate. The channel conductivity drops when the gate voltage rises (either positively or negatively, depending on whether the channel is constructed of P-type or N-type semiconductor material).

Enhancement mode is the second mode of operation for a MOSFET. When there is no voltage on the gate, the gadget does not conduct since there is no channel. The application of a voltage to the gate creates a channel. The better the device conducts, the higher the gate voltage.

Ques. Can MOSFET conduct in both directions? (1 Mark)

Ans. Yes, MOSFETs can conduct in both directions, hence they are bidirectional devices.

Ques. What are the operating regions of MOSFET? (1 Mark)

Ans. There are three operating regions of MOSFET, they are

Cut-off region
Ohmic region
Saturation region.

Ques. What are the key differences in construction between a MOSFET and a BJT? What impact do they have on a MOSFET's current conduction mechanism? (4 Marks)

Ans. A MOSFET, like a BJT, is made up of layers of p and n-type semiconductors that alternate. The p-type body area of a MOSFET, unlike the BJT, does not have an external electrical connection. A thin layer of SiO₂ protects the semiconductor from the gate terminal.

The source metallization shorts the body with an n+ type source. Minority carrier injection is thus prevented across the source-body contact.
The electric field produced by the gate-source voltage causes the creation of a high-density n-type channel in the p-type body area, which causes the MOSFET to conduct.
The n+ type source and drain regions are connected by this n-type channel.
Only the movement of electrons causes current to flow between the drain and the source through this channel (majority carriers).
Current conduction in a BJT is caused by minority carrier injection across the Base-Emitter junction. A MOSFET is a voltage-controlled majority carrier device, whereas a BJT is a bipolar device with a minority carrier.

Ques. What is Forward Transconductance, and how does it work? (2 Marks)

Ans. Forward transconductance (g_m) of a MOSFET is a measure of how much its drain current (i_d) changes with respect to changes in the gate-to-source voltage (V_GS), excluding the threshold voltage (V_GS(th)). Mathematically, it's represented as g_m = Δi_d / ΔV_GS.

Ques. What does "the channel is pinched off" mean? (2 Marks)

Ans. When VGS is larger than Vt, a channel is induced in a MOSFET. As VDS rises, current begins to flow from the Drain to the Source (triode region). When we increase VDto the point where the voltage between the gate and the channel at the drain end equals Vt, i.e. VGS − VDS = Vt, the channel depth at the drain end reduces to practically zero, and the channel is said to be pinched off. This is the point at which a MOSFET reaches saturation.

Ques. Write a short note on MOSFET. (3 Marks)

Ans. MOSFET stands for Metal oxide semiconductor Field Effect Transistor. MOSFETs are mainly used to amplify voltages in circuits. It consists of three terminals: Gate, Source, and Drain. Such transistors are of two classes and both of them are available as N-channel or P-channel:

Depletion Mode: The conductivity of the channel decreases when the voltage that passes through the gate terminal is either positive or negative. In the absence of such voltage, the conductivity is at its maximum.
Enhancement Mode: In the absence of voltage in this mode, there is no conductivity. The conductivity is enhanced whenever the gate terminal has maximum voltage.

Ques. What is a semiconductor diode? (2 Marks)

Ans. A p-n junction diode is a semiconductor diode. The current in such a diode only flows in one direction. There are 8 types of semiconductor diodes: Gunn diode, Photodiode, Variable capacitance diode, Zener diode, LED, Tunnel diode, Switching diode, and Rectifier diode.

Ques. Explain the working principle of MOSFET. (3 Marks)

Ans. The working of MOSFET depends upon the metal oxide capacitor(MOS). Other working principles of MOSFET are-

Metal oxide capacitor is the main part of the MOSFET.
The thin oxide layer is present between the source and the drain terminal.
MOSFET can be set from p-type to n-type by applying positive or negative gate voltages respectively.
When the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward through the substrate

Ques. How is MOSFET used as a switch? (5 Marks)

Ans. MOSFET is used as a switch is discussed below-

A N channel MOSFET is being used to switch the lamp for ON and OFF.
The positive voltage is applied at the gate of the MOSFET and the lamp is ON (B_GS=+v).
At the zero voltage level, the device turns off (V_GS=0).
If the resistive load of the lamp was to be replaced by an inductive load and connected to the relay or diode to protect the load.
When using MOSFET to switch either inductive load or capacitive load protection is required to contain the MOSFET applications.
If we’re not giving the protection then the MOSFET will be damaged.
For the MOSFET to operate as an analog switching device it needs to be switched between its cut region where V_GS=0 and saturation region where V_GS= +v.

For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates

Also Read:

Class 12 Physics Study Guides
Physics MCQs	Important Derivations in Physics	SI units in Physics
Physics Study Notes	Topics for Comparison in Physics	Class 12 Physics Practicals
Formulas in Physics	Class 12 Physics Book PDF	Topics with relation in physics
Class 12 Physics Notes	NCERT Solutions for Class 12 Physics	Important Physics Constants and Units

MOSFET: Construction, Principles and Types

Related Links

What is a MOSFET?

Construction of MOSFET

Working Principle of MOSFET