What is Field Effect Transistor (FET)? || Differences between BJT and FET || Types of FET

Summary

This video from the ALL ABOUT ELECTRONICS YouTube channel offers a concise introduction to Field Effect Transistors (FETs), highlighting their role in controlling current flow through an electric field. It differentiates FETs from Bipolar Junction Transistors (BJTs), emphasizing FETs' high input impedance, smaller size, and lower power consumption, making them ideal for integrated circuits and high-power applications. The video also touches on the two main types of FETs: JFETs, which use a reverse-biased PN junction for gate control, and IG-FETs, like the widely used MOSFETs, which feature an insulated gate. The script promises upcoming videos that will delve into the operation, symbols, and characteristics of these FETs.

Takeaways

• 📗 The Field Effect Transistor (FET) is a three-terminal device that controls current flow using an electric field.
• ⚡ FETs are widely used in integrated circuits, including computers, due to their efficient current control.
• 🔗 The three terminals of the FET are gate, drain, and source, with current flowing between drain and source, controlled by the voltage between gate and source.
• 💡 FETs are voltage-controlled devices, in contrast to Bipolar Junction Transistors (BJTs), which are current-controlled.
• 🔍 FETs are unipolar devices (relying on one type of charge carrier), while BJTs are bipolar devices (relying on both electrons and holes).
• ⚙️ FETs have high input impedance, making them ideal for use as buffer amplifiers, and are smaller and more power-efficient than BJTs.
• 📊 FETs come in two main types: Junction Field Effect Transistors (JFET) and Insulated Gate Field Effect Transistors (IG-FET), with MOSFET being the most common IG-FET.
• 🔋 The two types of MOSFETs are depletion mode (decreases charge carriers) and enhancement mode (increases charge carriers).
• 🔗 MOSFETs can be further classified into n-channel and p-channel types, based on the material used in the channel.
• 🧪 FETs are used in various applications, such as amplifiers, oscillators, analog switches, and in computing devices for low power consumption.

Q & A

What is a Field Effect Transistor (FET)?

A Field Effect Transistor (FET) is a three-terminal device that uses an electric field to control the flow of current through the device. It is widely used in many applications, including integrated circuits and computers.

What are the three terminals of an FET?

The three terminals of an FET are known as gate, drain, and source.

How does the current flow in an FET?

In an FET, the current flows between the drain and the source terminals, and this flow can be controlled by applying a voltage between the gate and the source terminals.

What is the difference between a FET and a Bipolar Junction Transistor (BJT) in terms of control mechanism?

FETs are voltage-controlled devices, where the input voltage between the gate and source controls the output current. In contrast, BJTs are current-controlled devices, where the input base current controls the output collector current.

Why are FETs considered unipolar devices?

FETs are considered unipolar devices because their operation relies on either holes or electrons, but not both, unlike BJTs which rely on both types of charges.

What is the significance of high input impedance in FETs?

The high input impedance of FETs makes them suitable for use as buffer amplifiers in many applications, as they can provide high resistance to the input signal without significantly altering it.

How do FETs compare to BJTs in terms of size and power consumption?

FETs are generally smaller in size compared to BJTs and have lower power consumption, which is why they are commonly used in integrated circuits and preferred in high power and computing applications where minimal power consumption is required.

What are the two main types of FETs discussed in the script?

The two main types of FETs discussed are the Junction Field Effect Transistor (JFET) and the Insulated Gate Field Effect Transistor (IG-FET), with MOSFET being the most common type of IG-FET.

How does a JFET control the flow of current through its channel?

In a JFET, the gate terminal is provided using a PN junction. When this PN junction is reverse-biased, the depletion region isolates the gate terminal from the channel, controlling the flow of current.

What is the difference between depletion and enhancement type MOSFETs?

In a depletion type MOSFET, the applied voltage at the gate terminal depletes the charge carriers in the channel, while in an enhancement type MOSFET, the applied voltage enhances the number of charge carriers, forming a channel between two n-regions.

What are the two classifications of MOSFETs based on the channel material?

MOSFETs can be classified as either n-channel or p-channel, depending on whether the channel is made of n-type or p-type semiconductor material.

Outlines

00:00

📡 Introduction to Field Effect Transistors (FETs)

This paragraph introduces the concept of Field Effect Transistors (FETs), emphasizing their importance in modern electronics, particularly in integrated circuits like computers. FETs are three-terminal devices that use an electric field to control current flow between the drain and source terminals, with the gate terminal's voltage controlling this field. The paragraph contrasts FETs with Bipolar Junction Transistors (BJTs), highlighting FETs as voltage-controlled devices with high input impedance, making them suitable for buffer amplifiers. It also touches on their smaller size and lower power consumption, which are advantages in high-power and computing applications. The paragraph concludes by differentiating FETs from BJTs in terms of unipolar versus bipolar operation and sets the stage for discussing the types of FETs.

05:05

🔬 Types of Field Effect Transistors

The second paragraph delves into the different types of FETs, starting with the Junction Field Effect Transistor (JFET), which uses a reverse-biased PN junction to isolate the gate from the channel. It explains the existence of n-channel and p-channel JFETs, depending on the semiconductor material used for the channel. The paragraph then introduces Insulated Gate Field Effect Transistors (IG-FETs), with a focus on the Metal-Oxide-Semiconductor FET (MOSFET), the most common IG-FET. It describes the structure of MOSFETs, which includes a metal gate and a silicon dioxide insulating layer. The discussion continues with the classification of MOSFETs into depletion and enhancement types, based on how the gate voltage affects the channel's charge carriers. The paragraph wraps up by mentioning that MOSFETs can be further classified as n-channel or p-channel and hints at other types of FETs like Fin-FET and CMOS, which will be covered in future videos.

Keywords

💡Field Effect Transistor (FET)

A Field Effect Transistor (FET) is a type of semiconductor device that uses an electric field to control the flow of current. It is a three-terminal device with gate, drain, and source terminals. In the video, FETs are described as being integral to modern integrated circuits, including computers, due to their ability to control current with an electric field, which is why they are called 'field effect' transistors. The script explains that by controlling the voltage at the gate, one can control the flow of current between the drain and source, making FETs voltage-controlled devices.

💡Gate

The gate in an FET is one of the three terminals, along with the source and drain. It plays a crucial role in controlling the flow of current by applying a voltage that generates an electric field, which in turn modulates the conductivity of the channel between the source and drain. The script mentions that the gate's voltage control is what differentiates FETs from BJTs, where current flow is controlled by the base current.

💡Drain and Source

The drain and source are the two terminals through which current flows in an FET. The script explains that the current flowing between these terminals can be controlled by the voltage applied to the gate. This is a fundamental concept in the operation of FETs, as it demonstrates how the electric field created by the gate voltage affects the current flow, which is the main principle behind the FET's function.

💡Electric Field

The electric field within an FET is generated by the voltage applied between the gate and the source. This field is what controls the flow of current through the device. The script emphasizes that controlling the electric field is the key mechanism by which FETs operate, hence the name 'field effect' transistor. The electric field's influence on the channel's conductivity is central to the FET's voltage-controlled nature.

💡Bipolar Junction Transistor (BJT)

A Bipolar Junction Transistor (BJT) is a type of transistor that contrasts with FETs in its operation. While FETs are voltage-controlled devices, BJTs are current-controlled, where the base current controls the collector current. The script highlights this difference, noting that BJTs rely on both types of charge carriers (electrons and holes), unlike FETs which are unipolar, relying on either electrons or holes.

💡Unipolar Device

A unipolar device, like an FET, operates using only one type of charge carrier, either electrons or holes. This is in contrast to bipolar devices like BJTs, which use both. The script explains that FETs are unipolar, which simplifies their operation and contributes to their high input impedance, a key advantage in certain applications.

💡Input Impedance

Input impedance refers to the measure of opposition to current flow at the input of a device. The script points out that FETs have a very high input impedance, which makes them suitable for use as buffer amplifiers. This high impedance is a result of the FET's unipolar operation and its reliance on controlling the electric field rather than the flow of current.

💡Integrated Circuits

Integrated circuits are complex electronic circuits fabricated on a single semiconductor material, often silicon. The script mentions that most integrated circuits, including those in computers, are designed using FETs. This is due to their small size, high input impedance, and low power consumption, which are all desirable characteristics in dense, power-efficient integrated circuits.

💡Power Consumption

Power consumption in the context of the script refers to the amount of energy used by a device. FETs are noted for having lower power consumption compared to BJTs, making them preferable for high power and computing applications where energy efficiency is critical. The script emphasizes this as an advantage of FETs over BJTs.

💡Junction Field Effect Transistor (JFET)

A Junction Field Effect Transistor (JFET) is a specific type of FET where the gate terminal is created using a PN junction. The script describes how in an n-type JFET, small p-type regions are fabricated near the channel to form a PN junction, which isolates the gate from the channel when reverse-biased. This structure allows the gate to control the current flow by modulating the depletion region, which is a key feature of JFETs.

💡Insulated Gate Field Effect Transistor (IGFET)

An Insulated Gate Field Effect Transistor (IGFET) is a type of FET that uses an insulating layer between the gate and the channel. The script introduces the IGFET and specifically mentions MOSFETs (Metal-Oxide-Semiconductor FETs) as the most common type. This insulating layer, typically made of silicon dioxide, allows for better control of the electric field and contributes to the high input impedance of IGFETs.

💡MOSFET

MOSFET stands for Metal-Oxide-Semiconductor Field Effect Transistor, which is a specific type of IGFET. The script explains that MOSFETs can be further classified into depletion type and enhancement type, depending on how the gate voltage affects the charge carriers in the channel. Depletion MOSFETs reduce the number of carriers with gate voltage, while enhancement MOSFETs increase them. This distinction is important for understanding the different behaviors and applications of MOSFETs.

Highlights

Introduction to the field effect transistor (FET), a three-terminal device used in most integrated circuits.

FET uses an electric field to control current flow between drain and source terminals.

The gate terminal in FET controls the current flow by applying voltage, creating an electric field.

FET is a voltage-controlled device, unlike the current-controlled Bipolar Junction Transistor (BJT).

FET is unipolar, operating on either electrons or holes, unlike the bipolar BJT.

FETs are used as amplifiers, oscillators, and analog switches in various applications.

FETs have a high input impedance, making them suitable as buffer amplifiers.

FETs are smaller and consume less power than BJTs, making them ideal for integrated circuits and high-power applications.

There are two main types of FETs: Junction Field Effect Transistor (JFET) and Insulated Gate Field Effect Transistor (IG-FET).

JFETs use a PN junction for the gate terminal, which can be n-channel or p-channel.

IG-FETs have an insulated layer between the gate and the channel, with MOSFET being the most common type.

MOSFETs can be depletion type, where voltage depletes charge carriers, or enhancement type, where voltage increases them.

Depletion type MOSFETs have a channel with fewer charge carriers when voltage is applied.

Enhancement type MOSFETs form a channel between n-regions when voltage is applied.

MOSFETs can be further classified into n-channel or p-channel types.

Other types of FETs include Fin-FET and CMOS, which will be discussed in future videos.

Upcoming videos will cover the working, symbols, and transfer characteristics of JFET and MOSFET.

The video concludes with an invitation for questions, suggestions, and engagement from viewers.

Transcripts

Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS. In this

video we will briefly learn about the field effect transistor. The field effect

transistor or FET is a three terminal device, which uses the electric

field to control the flow of current through the device. And it is very useful

in many applications. In fact today most of the integrated circuits including the

computers are designed using this FETs. So the three terminals of the FET

are known as gate, drain and source. so in FET, the current used to flow between

the drain and the source terminal. And this current can be controlled by

applying the voltage between the gate and the source terminal. So these applied

voltage generates the electric field within the device and by controlling

this electric field or in a way by controlling this voltage we can control

the flow of current through the device. so basically in this field effect

transistor, by controlling the electric field we can control the flow of current.

And that is why it is known as the field effect transistor. So this field effect

transistor is the voltage control device that means the input voltage between the

gate and the source terminal controls the output current. On the other end if

you look at the BJT or the Bipolar Junction Transistor, it is a current

controlled device, where the input base current controls the output collector

current. so this is the one of the differences but in the field effect

transistor and the bipolar Junction transistor. The second difference is the

FET is a unipolar device, while the BJT is bipolar device. Meaning that the BJT

relies on the two types of charges, the free electron and the holes. But the

operation of the FET relies on either holes or electron. Now in terms of the

application, the FETs are used in almost all the applications where the

BJTs are used. For example they are used as a amplifier or oscillator in many

applications and apart from that also used as analog switch in many

applications. But the biggest difference between the field effect transistor and

the BJT is that the input impedance of the field effect transistor is very high

and due to that they are used as a buffer amplifier in many applications.

apart from that the FETs are smaller in size compared to the BJTs. And

that's why they are commonly used in the integrated circuits.

Apart from that now if we talk in terms of the power consumption the power

consumption of the FET is less than BJT. And that's why they are preferred in

many high power applications as well as in the computing applications,

particularly where the required power consumption should be minimum. so these

are the few differences between the FET and the BJT. Now let us see the different

types of FETs. So basically there are two types of FETs. The first is

the junction field effect transistor or the JFET. And the second type is IG-FET.

or it is known as the insulated gate field effect transistor. And the MOSFET is

the most common type of IG-FET. so let us briefly discuss about these two

types of FETs. Now as I said earlier the FET has three terminals. The gate

source and the drain. And the current flows between the drain and the source

terminals. Now in this field effect transistor the path through which these

charge carrier flows is known as the channel and if this channel is made up

of n-type semiconductor then the field effect transistor is known as the

n-channel FET. Likewise, if the channel is made up of p-type material then it is

known as the p-channel FET. And in this FET, the gate terminal is placed

very close to this channel, so that it can control the flow of current through

this channel. now in this JFET this gate terminal is provided using this PN

Junction. So if you see the n-type JFET, two small p-type regions are

fabricated near this channel. And due to that the PN Junction is formed near this

channel and whenever this PN Junction is reversed bias then the depletion region

of this PN Junction isolates the gate terminal from the channel. And only a

small amount of reverse saturation current used to flow between these two

regions. so in a way this reverse bias PN Junction isolates the gate terminal from

the channel and that is why this type of field effect transistor is known as the

JFET or the junction field effect transistor. Now as I said earlier if this

channel is made up of n-type semiconductor then it is known as the

n-channel JFET. And likewise if it is made up of p-type semiconductor then the

JFET is known as the p-channel JFET. so in this way there are two types of

JFET. The n-channel and p-channel JFET. so now let us talk about the second type

of FET. So this IG-FET uses an insulated layer between the gate

terminal and the channel. And typically this insulated layer is formed from the

oxide layer of the semiconductor. now here the name IG-FET refers to

the any type of FET which has an insulated gate. And the most common form

of IG-FET is the MOSFET. So in this MOSFET, the gate is made up of

a metal layer and the insulating layer is made up of silicon dioxide.

now this MOSFET can be further classified into two types. The depletion

type and the enhancement type. so let us understand briefly about these two types.

now when we apply the voltage at the gate terminal then due to the electric

field it can either deplete or enhance a number of charge carriers in this given

channel. So by the application of the voltage if the number of charge carriers

gets depleted in this channel then it is known as the depletion type of FET.

and if the number of charge carrier increases then it is known as the

enhancement type of FET. So this structure which is shown in this

diagram is the depletion type of MOSFET where the applied voltage at the gate

terminal depletes the charge carriers in this n channel. While this structure

which is shown in the diagram is the enhancement type of MOSFET. so in this

type of MOSFET the channel is formed between these two n- regions whenever we

apply the voltage at the gate terminal. So these are the two types of MOSFETs.

and these two types of MOSFETs can be further classified either as n-channel

or p-channel MOSFETs. so these are the basic types of a FETs. And of course

there are other types of FETs like Fin-FET and the CMOS, but we will talk

about it in the separate video. so from the next video onwards we will see the

working of this JFET and the MOSFET. And we will also see the symbol as well as a

transfer characteristic of this different types of FETs. And once we

finish that then we will see that how to bias this FETs. So I hope in this

video you understood what is FET and what are the different types of FET.

So if you have any question or suggestion, do let me know here in the

comment section below. If you liked this video, hit the like button and subscribe

this channel for more such videos.