Field-effect Transistor is a type of transistor that Controls the flow of current with the help of the voltage supplied to its gate terminal. It is commonly used for the amplification of the signals, mostly it amplifies both analog and digital signals. FET has three basic terminals which are known as.

• Base: charge careers leave the FET through the base, in other words, it is the exit point of charge careers.

• Source: As the name represents it allows the charge carrier to enter into the FET transistor.

• Gate: Gate plays the role of the switch and regulates the charge flow between source and Base.

As BJT transistors are bipolar because they have holes and free electrons present in them to pass electric signals, but FET is not like that they are unipolar so either they use holes or free electrons in their circuit which means in FET flow of electric current is only due to majority charge career or minority charge career.

Symbol:

 Typically the FET is represented by the symbol below. Shoeing three different terminals which are attached in the circuits accordingly.

As per characteristics, they are divide into two most common types known as JFET and MOSFET and each divides into two categories known as P- channel and N- channel.

Working of FET:

Conduction of electric impulses is accomplished using either holes or free electrons, as previously stated. The gate terminal is connected to the source voltage, which controls the current flow between the source and the base.

The “channel” is defined as a pathway between the source and the base, which either contain an N-type or P-type junction.

• N-type Channel:    When a negative source is given to the gate, the depletion area expands on both sides, allowing the electric current to flow through the channels despite the fact that there is very little space for the charge to travel. More the voltage, results in more the depletion region. When we kept increasing the negative voltage, the channel reached a point where no more current could pass through it, that point is known as “pinch –off “and the voltage is known as “pinch-off voltage “. The flow of current through the channel and the depletion region is shown in the figure below.

Conversely, in “enhancement mode” or Conduction mode positive Source is applied to the base which increases the conductivity of the electric signals as in this mode electrons are attracted towards the base side and the depletion region starts decreasing and eases the flow of electric current through the channel. The source voltage which is necessary for this process is known as “Threshold voltage”.

• P-type Channel:

As in N-type FET, P-type FET shows the same behavior of modes which are depletion mode and conductivity mode. In the “depletion-mode “A positive source is attached to the gate terminal which attracts the electrons towards the gate and leaves positive ions behind, this process widens the depletion region and enhances the conductivity. The flow of electric charges are visible in the figure below when negative terminal source is attached with the Gate terminal.

Types of Field Effective Transistors:

Field effective transistors are divided into various categories according to their operational behavior application in circuits, but the most important types are MOSFET and JFET. 

• JFET:

 The working of JFET is the same as we discussed earlier its conductivity depends upon the reduction and formation of the depletion region. It has both N-type and P-type channel working procedures to pass out the electric charge.

• MOSFET:

     The working principle of MOSFET is such that it induces an insulation layer between the gate and the channel. As its name represents Metal oxide semiconductor Field effective Transistor. This layer acts as a dielectric between the gate and the channel. It has two modes of working which are enhancement mode and depletion mode and is mostly used for the enhancement of electric signals.

• Dual Gate MOSFET:

    This type of MOSFET contains two insulation layers between Gate and channel, this alteration provides further perfection in the Performance of transistor than single gate transistor. These types of transistors are mostly used for multiplication or mixing.

• MOSFET:

    MESFET is known as a Metal silicon Field effective transistor. They are mostly used in RF technologies, but they are very sensitive to tackle as they may damage handling. They are useful where we need high gain Low noise performance, high input impedance, and low capacitance. They are mostly used in military appliances especially radar and Sonar technologies

• HEMT:

    HEMT is commonly known as high electron mobility transistor, their working principle is the same just as FET, but they are very much expensive and mostly used where high performance and affectivity is required. As the main charge carriers in them are majority charge carriers so they are mostly used in high-speed switching and amplification of electronic signals. 

• FINFET:

    Finder is the most modern technology in transistors as they are mostly used for high-level integration. They are difficult to handle because of their sensitivity and size. Most modern complex integrated circuits use these types of technologies.

• Pros and Cons of FET;

AS compared to bipolar transistors FET has some advantages and disadvantages, it is mostly preferred for weak signals like in communications or broadcasting. They are mostly used in those circuits where high impedance is required, these types of transistors are used in circuits by applying them on I.C chips.

One of the few drawbacks of the unipolar transistor is that they often show low-frequency responses because of their high impedance and capacitance. Bipolar transistors are far better in terms of linearity where FET shows poor linearity in circuits.

Applications:

• Used as low noise amplifier
• Buffer amplifier especially for radio signals
• As a switch
• Used in Computer and electronic chipboards.
• Cascade amplifier
• Used as multiplexer
• Used as a Phase shifter for several IC circuits
• Current limiter
• Used as chopper
• Used for radio frequencies and TV signals, especially in military equipment
• Used in large scale integration in mostly computer-related devices