In this article  we will  learn about the field effect transistor(FET) .

1. What is field effect transistor? 

2. Types of FET 

3. Application of FET 

4. Why it is known as field effect transistor? 

5. Junction field effect transistor (JFET) 

6. Metal oxide semiconductor field effect transistor (MOSFET) 

7. Difference between bipolar junction transistor (BJT) and field effect transistor  (FET )

   

Field effect transistor notes


What is FET (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. 

         it is very useful in many applications. In fact today most of the integrated circuits including the computers are designed using this FETs. 

Field effect transistor (FET)

characteristics and working of FET 

      The three terminals of the FET are known as 

1.gate

2. drain

3. 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.

   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.

Difference between BJT and FET 

  1. 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. 

    2.  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. 

3. 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.

4. 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. 

Application of FET (field effect transistor) 

        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.        

         FETs are smaller in size compared to the BJTs. And that's why they are commonly used in the integrated circuits. 

 Types of field effect transistor (FET)     

       Basically there are two types of FETs.  

1. junction field effect transistor (JFET)

2.  IG-FET. or it is known as the insulated gate field effect transistor (MOSFET is the most common type of IG-FET)

       so let us discuss about these two types of FETs. Now as we know  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. 

JFET (junction field effect transistor) 

     

Junction field effect transistor (JFET)

In 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 PNJunction 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 effecttransistor

Types of JFET 

     there are two types of JFET. 

 1. n-channel JFET

2. p-channel JFET.

      Now as we discuss 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.

IG-FET 

       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.

MOSFET

         

MOSFET

So in this MOSFET, the gate is made up of a metal layer and the insulating layeris made up of silicon dioxide

          this MOSFET can be further classified into two types. 

   1. The depletion type

    2. 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 above 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 inthis n channel. 

     

Enhancement type MOSFET

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 discuss about it in the separate article . 


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