Field Effect Transistors(FET)
1 Answer


  • The field effect transistor(FET) is three terminal device.

  • It is also known as 'unipolar' transistor

  • FET has many applications as that of BJT's

  • Following table shows comparative study of BJT and FET

Parameters BJT(Bipolar) FET(Unipolar)
1 No. of terminals and their names Three(Emitter, Base, Collector) Three(source,gate,drain)
2 Control Current control device[As output collector current is function of input base current] Voltage control device [As output current is function of input voltage VGS]
3 Input impedance Comparatively low High
4 Thermal stability unstable(due to thermal runaway process) Stable
5 Types pnp and npn JFET and MOSFET

Q) Why the FET's are considered as unipolar device?

  • In FET, current is present due to only one type of charge carriers. i.e. only 'electrons' or 'holes'. It's operation depends only on majority charge carrier.

  • Hence, FETs are considered as 'unipolar' devices.

Q) State advantages and disadvantages of FET.

There are some advantages of FET over BJT.

They are:

  • It is immune to radiation.

  • It has very high input impedance.

  • It has better thermal stability.

  • Less noisy.

Disadvantages of FET are:

  • Small gain-bandwidth product

Types of field effect transistors

Broadly speaking, there are two main types of field-effect transistor

a) Junction field effect transistor(JFET)

b) Metal oxide semiconductor FET(MOSFET)

It is also known as Insulated Gate FET.(IGFET)

Following chart shows further classification of FET.

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  • It is Metal Oxide semiconductor Field Effect Transistor.

  • MOSFET has following types:

a) Depletion type MOSFET(D-MOSFET)

b) Enhancement type MOSFET(E-MOSFET)


D-MOSFET further classified as

  1. n-channel D-MOSFET

  2. p-channel D-MOSFET

Q) Describe N-channel D-MOSFET

(A) Construction:

  • Following figure shows the basic structure of N-channel depletion type MOSFET.

enter image description here

  • It consists of conducting bar N-type material with an insulated gate on the left and p-region on the right.

  • Free electron can flow from source to drain(D) through the N-type material.

  • p region is called as substrate.

  • A layer of insulating material(i.e.S$_{i}$O$_{2}$) is deposited on left of channel.

(B) Working:

  • When V$_{GS}$ = 0V

  • Usually fourth terminal made of MOSFET i.e. substrate is made common with source terminal.

  • Following figure shows biasing condition for V$_{GS}$ = 0V.

  • In figure below, gate, source, and substrate are connected together to ground pin to make V$_{GS}$ = 0V.

  • A positive voltage V$_{DS}$ is applied between drain to source.

enter image description here

  • Due to the positive voltage applied to the drain terminal, free electrons from the channel are attracted to drain and the drain current starts flowing.

When V$_{GS}$ = negative[Depletion mode of operation]

  • Due to applied negative voltage between gate and source, gate terminal will repel free electrons towards substrate and attracts holes from substrate.

  • Those electrons and holes will get recombined inside the channel and reduces width of channel and amount of current.

  • Thus, with increase in negative value of V$_{GS}$, there is decrease in amount of current.

enter image description here

When V$_{GS}$ = positive[Enhancement mode of operation]

  • If gate voltage is made positive w.r.t source, this will increase the no. of free electron passing through channel.

  • The greater the gate voltage, greater is the no. of free electrons passing through channel.

Characteristics of D-MOSFET

a) Drain characteristics(o/p characteristics)

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b) Transfer characteristics

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Circuit symbol of n-channel D-MOSFET is as shown below:

enter image description here

Q) Describe p-channel D-MOSFET

  • Construction

The basic construction of a p-channel D-MOSFET is similar to that of n-channel except that the conducting bar is of p-type material and the substrate is of n.type material.


enter image description here


  • It is enhancement type of MOSFET

  • It operates in enhancement mode.

  • It differs from D-MOSFET that it has no physical channel.


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The basic construction of n-channel enhancement is shown in figure below:

  • It consists of p-type semiconductor material as a substrate.
  • The drain and source terminal are connected to the n-type doped region through metallic material.
  • The insulating layer of S$_{i}$O$_{2}$ is present on left side of substrate.


The operation can be explained with two different operating conditions:

  • with V$_{GS}$ = 0V

  • with V$_{GS}$ = positive

with V$_{GS}$ = 0V

  • It is shown in figure below:

enter image description here

  • When V$_{GS}$ = 0V and positive voltage V$_{DS}$ is applied between drain to source, due to absence of channel , there is no any flow of current.

i.e. When V$_{GS}$ = 0V, I$_{D}$ = 0mA.

When V$_{GS}$ = positive

enter image description here

  • The positive voltage at gate terminal will repel the holes present in substrate.

  • This will create a layer of electrons near S$_{i}$O$_{2}$.

  • As we increase V$_{GS}$, the no. of electrons will also increase near S$_{i}$O$_{2}$.

  • With this, there is creation of channel. So, drain current starts flowing through this induced channel.


enter image description here

1 An insulating oxide layer(S$_{i}O_{2}$) is present between gate and channel The insulating oxide layer is present between gate and substrate.
2 n or p type channel is present Channel is not present. At the time of operation, an induced channel gets created.
3 For an n channel DMOSFET, the V$_{GS}$ can be negative for depletion mode and positive for enhancement mode. For an n channel EMOSFET, V$_{GS}$ will be only positive.
4 For an n channel DMOSFET, I$_{D}$ decreases as V$_{GS}$ becomes more and more negative For an n channel EMOSFET, I$_{D}$ increases as V$_{GS}$ becomes more and more positive
5 For an n channel DMOSFET, I$_{D}$ = 0 for $\mid V_{GS} \mid \ge V_{P}$. For an n channel EMOSFET, I$_{D}$ = 0 for V$_{GS} \le V_{T}$.
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