What is MOSFET?

In the world of electronics, you all must have heard the term Transistor in your life at least once. But have you ever thought about what are transistors? How do they work? What are the types of transistors? So today you will get the answer to all such questions. In this article, we will understand about the device MOSFET and its characteristics. But before that let me clear the term transistor.

So a transistor is a type of semiconductor device that is used to maintain and regulate the voltage and current level. It has the functions of a switch and an amplifier. You might be thinking about what is the need to study transistors. This is because it is one of the essential components in most of the electronic devices present today. We cannot imagine this world of technology without transistors.

MOSFET Basics

MOSFET is a very popular kind of IG-FET. The full form of MOSFET is the Metal Oxide Semiconductor Field Effect Transistor. The diagram of MOSFET is given below.

In these kinds of devices, the gate terminal is separated from the channel using the insulating layer. This insulating layer is formed from the oxide layer of the semiconductor. The insulating layer of MOSFET is formed from SiO₂.

What is MOSFET?

Now MOSFET is also classified into two types:-

1. Depletion type

2. Enhancement type

When we provide external voltage in the channel, it can either increase or decrease the amount of charge carriers in the channel. If the number of charge carriers increases, it is known as enhancement type MOSFET. But if the number of charge carriers decrease then it is known as depletion type MOSFET.

What is FET ?

Now one of those transistors is FET. The full form of FET is a Field Effect Transistor. It is a three-terminal device that uses the electric field to regulate and maintain the flow of current. The three terminals are :

1. Gate
2. Drain
3. Source

Current flows between two terminals, drain and source. The flow of current can be controlled by applying external voltage between gate and source. This external voltage generates electric field in device. So by controlling electric field and voltage, we can regulate the flow of current. Thus it is a Voltage Controlled Device. It has many applications such as Integrated circuits, oscillators and buffer amplifiers. They are small in size and because of this they are used in ICs.

Now there are two types of FET:-

1. JFET ( Junction Field Effect Transistor)

2. IG-FET (Insulated Gate Field Effect Transistor)

Important Terminologies

• Transistor : It is a kind of semiconductor device which can generate electric current or voltage on its own.
• Gate Terminal : The current conduction between drain and source is controlled by applying voltage in the gate terminal.
• Threshold voltage : Maximum amount of voltage required for the formation of channel.
• p-channel : Channel made with p-type semiconductor.
• n-channel : Channel made with n-type semiconductor.
• Saturation : Level in which something becomes constant.

MOSFET Types

Now these enhancement type and depletion type MOSFET is further classified into p-channel and n-channel MOSFET.

Types of MOSFET

Enhancement Type MOSFET

• In this type of MOSFET, no channel is present from the beginning and hence no current flows.
• But when the +ve voltage more than the threshold voltage is applied, it leads to an enhancement of a channel between the drain and source due to the gate voltage and thus it results in the conductivity of device.
• They are mostly used in digital applications.

Depletion Type MOSFET

• In this type of MOSFET, channel is present from the beginning which means conduction of current is there.
• -ve voltage is applied to reduce the flow of current and this reduces the width of the channel.
• Not used commonly in devices.

Working Principle of MOSFET

MOSFET is a type of transistor in which conductivity depends upon the semiconductor channel across the drain and source terminal. This semiconductor channel may be p-channel or n-channel depending upon the configuration of the MOSFET.

A MOSFET consists of three terminals- drain, source and gate. By applying some voltage across gate and source, there forms a inversion layer or a channel between the drain and source if the voltage applied is threshold voltage. (Threshold voltage is the minimum required voltage for the conduction of current) . If the applied voltage is less than the threshold voltage, no channel is formed. Hence current cannot flow in the MOSFET. This situation is called as Cutoff region (OFF).

And after a certain level of voltage, the current becomes constant in the MOSFET. This condition is called as saturation point. MOSFET is a voltage controlled device so the thickness of channel and the amount of current depends upon the voltage applied across gate and source. If more voltage applied, width of channel increases and more amount of current able to flow through the device.

MOSFET Construction

MOSFET is a transistor which is used as switch or amplifier and in many other applications. The basic construction of MOSFET can be explained as below:-

• Substrate : MOSFET is constructed on a silicon wafer that is it acts as a base of the device.
• SiO₂ : A thin layer of insulating material is formed with SiO₂ for the exchange of electrons and holes.
• Gate Terminal : A gate terminal is formed on the insulating layer. This controls the flow of current between the drain and source with the help of gate voltage.
• Source and drain terminals : These are created on the either side of the gate. These are basically doped regions.
• Channel : Region between the gate, drain and source is known as channel which controls the flow of charge among them.

Construction

In the above figure, gate ,source and drain are he three terminals. Below are the two channels and body terminal.

Characteristics of MOSFET

MOSFETs have majorly two characteristics:-

1. Drain characteristics

2. Transfer characteristics

Enhancement Type MOSFET

1. Drain characteristics of Enhancement Type MOSFET

Characteristics between output current and output voltage. O/P i -> I_D, O/P v -> V_DS , control variable = V_GS. Here we will plot a graph between I_D and V_DS for various levels of V_GS.

Case 1: V_GS1 > V_T

(Here V_T is the threshold voltage)

V_eff = V_GS1 – V_T

• will directly affect the width of the n-channel
• significant amount of drain current will flow through it.

When V_DS is increased to a certain level, drain current becomes constant. This is called pinch-off condition.

V_DS + V_GD – V_GS = 0

V_GD = V_GS – V_DS

V_GD = V_T , pinch off voltage

Case 2: V_GS1 > V_GS2

conductivity of 1 > conductivity of 2

R₂> R₁

slope of 2 < slope of 1

Now there are few regions in this graph:-

1. Saturation region : Region in which drain current is constant.
2. Triode region : Left area of locus of V_{DS saturation}.
3. Cut-off region : When it can’t achieve the threshold voltage, MOSFET remains OFF. Current is 0 amp.

   

With increasing voltage, current flowing through the terminals increases with voltage. You can see this in the graph. V_GS more than the threshold voltage is the condition of flowing current.

2. Transfer Characteristics of Enhancement Type MOSFET

Characteristics between output current and input voltage.

V = V_T (Threshold Voltage)

• When the value of voltage across gate and source is less than the threshold voltage

Working of N-channel Enhancement Type MOSFET

In such kind of MOSFET, we have to make gate terminal more +ve, hence +ve charges will accumulate in the gate and will attract -ve charges in the body. Electrons will be drifted towards the surface and the region near the surface will become less p type. +ve charges are pushed down. So above region will become n type and thus a channel is formed.

If we will increase the voltage between gate and source, then the width of channel will increase. And if the voltage is more than a particular voltage, the channel width is sufficient to allow flow of current. And this particular voltage is called Threshold voltage. And the resultant current is known as drain current.

N-channel enhancement type

Working of P-channel Enhancement Type MOSFET

A p-channel enhancement type MOSFET is a type of MOSFET which works by applying +ve voltage to the device. It operates according to the voltage applied to the gate terminal. Hence flow of current occurs between drain and source. Step by step explanation:-

• It has mainly three terminals, source, drain and gate. It forms on a semiconductor substrate.
• MOSFET has a thin insulating layer of oxide in it to separate it from the semiconductor. The source and drain terminals are made with p type semiconductor and so is the enhanced channel.
• In a p channel MOSFET, majority charge carriers are the holes.
• It is generally OFF when no voltage is applied. On applying voltage, it creates an electric field in the oxide layer and hence a channel is formed by the movement of holes.
• Then current starts flowing in the channel between drain and source.
• Substrate/body terminal is made up of n type semiconductor.
• Current in the channel can be controlled by gate voltage.

P channel enhancement type

Working of N- channel Depletion Type MOSFET

• In this type of MOSFET, three terminals are present- drain, source and gate.
• Same as other MOSFETs, it also contains a thin insulating layer made up of gate oxide to avoid direct contact with the metal.
• Here, channel is present from the beginning. By applying a +ve voltage it creates a depletion region reducing the charge carriers and results in decrement of current.
• MOSFET is generally ON.
• Drain and source terminal is made up of n-type semiconductor.

N channel depletion type

Working of P- channel Depletion Type MOSFET

Like any other MOSFETs, this is also made with three main channels- drain, source and gate. Step by step explanation:-

• Similar to other MOSFETs, there are mainly three terminals- gate, source and drain.
• The current between drain and source is controlled by the gate voltage but there is a difference. Channel is present from the beginning when no voltage is applied.
• Source and drain terminals are made with p type semiconductor and so is the channel. Majority charge carriers are the holes.
• Applying a -ve voltage, the channel depletes and hence he width of the channel decreases and hence the resistance increases decreasing the amount of current flowing.
• It is a voltage controlled resistor.

P channel depletion type

Symbols

n-channel E-type

Symbol of n-channel enhancement type.

p-channel E-type

Symbol of p-channel enhancement type.

Depletion Type MOSFET

Drain characteristics : I_D V/S V_DS for various V_GS.

Drain characteristics

Transfer Characteristics : I_D V/S V_GS for fixed V_DS.

Transfer characteristics

Symbols

N-channel D-type

Symbol of n-channel depletion type.

P-channel D-type

Symbol of p-channel depletion type.

Difference Between Enhancement Type and Depletion Type MOSFET

Enhancement Type MOSFET	Depletion Type MOSFET
1. By applying external voltage in the channel, if the amount of charge carriers increases, it is known as enhancement type MOSFET.	1. By applying external voltage in the channel, if the amount of charge carriers decreases, it is known as depletion type MOSFET.
2. No channel is present in the beginning.	2. Channel is present from the beginning.
3. There is a threshold voltage.	3. There is no such threshold voltage.
4. It does not produce current in the absence of VGS.	4. It can produce current without any gate voltage.

Uses of MOSFET

• Used in digital logic circuits.
• It is used as Amplifiers
• They are used in Integrated circuits due to small size.
• It is also used in Microprocessors
• Used in Power electronics

Operating Regions of MOSFET

1. Cutoff Region: In this region of MOSFET, no current flows as the voltage applied in the MOSFET is less than the threshold voltage resulting in the failure of formation of oxide layer. State is OFF.
2. Saturation Region: In this region of MOSFET, a constant amount of current flows between the drain and source because of the threshold voltage. State of MOSFET is ON in this case.
3. Triode Region: It is known as partially conducting state. It is not fully turned on but there is a channel for current flow. Voltage here is moderate.

Switching characteristics for both N channel and P channel MOSFET in tabular form

MOSFET	VGS < 0	VGS = 0	VGS > 0
1. P-channel enhancement type	ON	OFF	OFF
2. N-channel enhancement type	OFF	OFF	ON
3. P-channel depletion type	ON	ON	OFF
3. N-channel depletion type	OFF	ON	ON

Applications of MOSFET

• Amplifiers: MOSFETs are used as an amplifiers in order to amplify weak signals.
• Switching power supplies: They are used as switches because they can alter power supply efficiently.
• Digital logic gates: They are used to build logic gates such as NAND, NOR etc.
• Voltage regulators: They are used as voltage regulators because they can control the amount of voltage.
• Memory devices: They are used in memory cells.

Advantages of MOSFET

• High Switching Speed : It can change its state rapidly from ON and OFF position.
• Low Power Consumption : Useful for battery operated devices.
• High input impedance : They use minimum input current.
• Low noise : They produce low noise and do not cause much disturbance.

Disadvantages of MOSFET

• Breakdown: MOSFETS have a very thin layer of gate oxide so on applying high voltage it can cause to breakdown of entire device.
• Temperature: Some functions of MOSFET are altered because of varying temperatures.
• Voltage capacity: They have a limited voltage capacity.
• Expensive: Complex manufacturing of MOSFETS can lead to high cost of overall electronic device.

Conclusion

So in this article, we studied about MOSFETs in brief. Its drain and transfer characteristics and how it plays a pivotal role in making different type of electronic devices. We also saw the working of depletion and enhancement type MOSFETs, its differences and symbols. By referring this article, you can get an overview of MOSFET and rest you can also explore it on the internet as it is a vast topic.

Frequently Asked Questions on MOSFET – FAQs

What is active device?

Any type of device which has the ability to control the flow of electrons is called as active device. Eg-BJT, MOSFET, etc.

Why do we use SiO₂ in MOSFET?

We use thin layer of SiO₂ because we want the control over surface by gate electrode.

What do you mean by the term inversion?

Changing n type into p type by making gate terminal more positive with respect to substrate or body terminal.