Field Effect Transistors (FET)

It has 3 terminal device just like BJT(bipolar junction transistor). It is also a semiconductor device. FET can be turned on and off by electric field.

Two types of FET:

  • JFET (Junction Field Effect Transistor) or FET
  • MOSFET(Metal Oxide semiconductor Field Effect Transistor) or IGBT(Gate Insulated Field Effect Transistor)

First we discuss about FET or JFET

JFET are of two types i.e. P channel or N channel

Construction of FET/JFET:

First we dicuss about the construction details of N channel , than will take P channel

From the picture below, you see in the N channel FET, there is a N type semiconductor and on this Ntype semiconductor , two P type area is created by diffusion. Two p type area created on this N type semiconductor so two PN diode is formed ..the junction between these is called gate or simply junction.

The area between these two gate is called Channel. So is the name of this FET, N channel is kept.

Both the P areas on this N channel FET are joined together and a single terminal comes out known as gate G.

The two opposite ends of N channel FET is known as drain D and souce S.

Similar is the construction of P channel type FET. Two N areas are formed by diffusion on a P type semiconductor.

Working of FET or JEFT:

We will dicuss here N channel FET working as it is mostly used.

So, a JEFT or FET has three terminals i.e. Gate , Drain and Source.

To understand the working , we need to give this FET electric supply.

The souce terminals is made common between input and output and there are now two circuits i.e. input circuit gate and source circuit and output circuit drain and source circuit .

A battery VGG is connected between gate and source to make the reverse bias.

Similarly a battery VDD is connected between drain and source in the forward bias.

Due to reverse bias at gate, the depletion layers are formed and extended towards drain terminal. Higher the voltage at gate, higher the reverse bias and greater extension of reverse bias towards drain and the Channel becomes narrow.

And due to the battery VDD, as it forward biases the drain and source terminals, the current flows from drain to source called drain current Id.

This drain current causes voltage drop along the channel and its voltage drop towards drain is more than source.

So depletion region is more penetrated towards drain terminal.

Maximum Drain current : maximum drain current flows between drain to source when there is no voltage at gate. As there is no voltage at the gate .there is no depletion region formed in the channel and whole channel is clear and wider.

Zero drain current or minimum drain current: when voltage at gate raised and raised than two depletion regions becoming more and more and than touch each other than channel is blocked and no current flows through the channel and drain current becomes zero.

As you see, the maximum drain current flows when gate voltage is zero and gate current is zero when gate voltage is maximum. So voltage at the gate controlles the output drain current. So FET is voltage controlled device.

Advantages of FET/JFET over BJT:

  • BJT is current controlled device while FET is voltage controlled device .
  • Noise in BJT is more than FET.
  • FET has better thermal stability over BJT.
  • BJT has low input impedance ( input circuit foraward bias) and FET has high input impedance (Input circuit reverse biased)

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