[Physics Class Notes] on FET Transistor Pdf for Exam

Fet Transistor stands for Field-Effect transistor. The field-effect transistor (FET) is a type of transistor that controls the flow of current in a semiconductor using an electric field. 

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FETs are three-terminal devices with a source, gate, and drain. The application of a voltage to the gate, which modifies the conductivity between the drain and source, controls the flow of current in FETs.

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The first patent for FET transistors was filed by Julias Edgar in 1926. Since then much development has taken place. Another patent was filed by Oskar Heil in 1934. The junction gate that is used in field-effect transistors was created at the Bell Labs by William Shockley. Many other advancements in FET Transistors have been made over the years. 

Working of FET Transistor

The Fet transistor is a voltage-operated device in which the voltage applied is used to control the current flowing. It is also known by the name unipolar transistor as they undergo an operation of a single-carrier type. The input impedance is high in all forms and types of FET. The conductivity is always regulated with the help of applied voltage from the field-effect transistor’s terminal. Moreover, the density of the carrier charge affects conductivity. 

A FET transistor is a device with three major components: Source, Drain, and Gate. The source is one of the terminals of the FET transistor through which most of the carriers enter the bar. The Drain is the second terminal through which the majority of carriers lead the bar. The Gate has two terminals that are internally connected with each other. 

Since the gate in a FET transistor is reverse biased, the gate current is practically zero. The drain supply is connected to the source terminal leading to the electrons flow which provides the necessary carriers.  

FET Transistor – Types and Its Working Principles

There is another subdivision of FET Transistors. In one of the types, the current is taken up primarily by the majority carriers and is therefore called majority charge carrier devices. There are minority charge carrier devices, as well,  in which the current flow is primarily due to minority carriers. 

The two terminals, source, and gate have a potential between them which in turn has the conductivity of the channel as a function of it. The three terminals i.e. source, drain, and gate are there for every FET Transistor. The function of the gate terminal is similar to the gate in real life as the gate can open and close and can either choose to permit the passage of electrons or stop them altogether. 

FETs are categorised as:

1. Junction Field Effect Transistor (JFET)

2. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

1.Junction Field Effect Transistor (JFET)

The Junction FET transistor is a form of field-effect transistor that can be used to control a switch electrically. Between the sources and the drain terminals, electric energy travels through an active channel. 

The channel is strained and the electric current is switched off by supplying a reverse bias voltage to the gate terminal.

Working Principle:

The working of these JFETs is based on the channels that form between the terminals. Either an n-type or a p-type channel can be used. It’s called an n-channel JFET because it has an n-type channel, and it’s called a p-channel JFET because it has a p-type channel.

FET transistors are made in the same way as N-P-N and P-N-P transistors are made in BJT (Bipolar Junction Transistor). These JFETs have a channel that can be either n or p-type.

• It is classified as an n-channel JFET or a p-channel JFET depending on the channel.

• The source terminal connects the positive side of an n-channel JFET.

• In this n-channel JFET, the drain terminal has the largest potential compared to the gate.

• The connection created by the drain and gate terminals is in reverse bias. As a result, the depletion region around the drain is wider than the source.

• The majority of the charge carriers, which are electrons, flow from the terminal drain to the source.

• As the potential at the drain rises, the flow of carriers rises with it, and the flow of current also rises with it. 

• However, when the voltages at the drain and source are increased to a particular level, the current flow is stopped.

• The JFET is well-known for its ability to control current through the application of input voltages. In this transistor, the input impedance is at its highest point.

• There is no current evidence at the gate terminal when the JFET is in its optimum mode.

That is how an n-channel JFET operates. Only a change in the polarities of the supplies causes the FET to operate as a p-channel JFET.

2.Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFETs work by applying a voltage to channels that already exist or form. MOSFETs are classified into two types based on their operation modes: 

In the enhancement mode, the gate voltage induces the channel, whereas, in the depletion mode, the MOSFET operates owing to the existing channel.

There are two types of MOSFET depletion models: n-type and p-type. The only difference is the substrate deposition. The formation of the depletion zone is caused by a concentration of carriers that are preferred by the majority. Conductivity is affected by the width of the depletion.

A channel is formed in the enhancement mode when a voltage applied to the gate terminal exceeds a threshold voltage. It could be n-type for a P-type substrate and p-type for an N-type substrate. The enhancement mode is classified as N-type Enhancement MOSFET or P-type Enhancement MOSFET based on the channel formation. MOSFETs of the enhancement type are more commonly used than those of the depletion type.

Difference Between FET and MOSFET

The main difference between the two major types of FET transistors – JFET and MOSFET- is that JFET (Junction Field Effect Transistor) is a three-terminal semiconductor device while MOSFET (Metal oxide semiconductor field-effect transistor) is a four-terminal semiconductor device. JFET can only operate in the depletion mode. While MOSFET can operate in the enhancement as well as the depletion mode. The input impedance is higher in MOSFET making them more resistive. In comparison to the price, MOSFET is more expensive than JFET. 

Due to high input impedance, FET transistors are commonly used in and as input amplifiers in electronic voltmeters, oscilloscopes, and other measuring devices. T
hey also occupy little space which makes them more efficient for other devices. 

Conclusion

The article covers some important and key characteristics of FET Transistors. This foundational knowledge can be further used in understanding more concepts related to electricity and current. The definition of FET, types of FET, and how it regulates the circuits are the key highlights of this article.