Field Effect Transistor

An FET is a three-terminal unipolar semiconductor device. It is a voltage controlled device unlike a bipolar junction transistor. The main advantage of FET is that it has a very high input impedance, which is in the order of Mega Ohms. It has many advantages like low power consumption, low heat dissipation and FETs are highly efficient devices. The following image shows how a practical FET looks like.

The FET is a unipolar device, which means that it is made using either p-type or n-type material as main substrate. Hence the current conduction of a FET is done by either electrons or holes.

Features of FET

The following are the varied features of a Field Effect Transistor.

· Unipolar − It is unipolar as either holes or electrons are responsible for conduction.

· High input impedance − The input current in a FET flows due to the reverse bias. Hence it has high input impedance.

· Voltage controlled device − As the output voltage of a FET is controlled by the gate input voltage, FET is called as the voltage controlled device.

· Noise is low − There are no junctions present in the conduction path. Hence noise is lower than in BJTs.

· Gain is characterized as transconductance. Transconductance is the ratio of change in output current to the change in input voltage.

· The output impedance of a FET is low.

Advantages of FET

To prefer a FET over BJT, there should be few advantages of using FETs, rather than BJTs. Let us try to summarize the advantages of FET over BJT.

JFET	BJT
It is an unipolar device	It is a bipolar device
Voltage driven device	Current driven device
High input impedance	Low input impedance
Low noise level	High noise level
Better thermal stability	Less thermal stability
Gain is characterized by transconductance	Gain is characterized by voltage gain

Applications of FET

· FET is used in circuits to reduce the loading effect.

· FETs are used in many circuits such as Buffer Amplifier, Phase shift Oscillators and Voltmeters.

FET Terminals

Though FET is a three terminal device, they are not the same as BJT terminals. The three terminals of FET are Gate, Source and Drain. The Source terminal in FET is analogous to the Emitter in BJT, while Gate is analogous to Base and Drain to Collector.

The symbols of a FET for both NPN and PNP types are as shown below

Source

· The Source terminal in a Field Effect Transistor is the one through which the carriers enter the channel.

· This is analogous to the emitter terminal in a Bipolar Junction Transistor.

· The Source terminal can be designated as S.

· The current entering the channel at Source terminal is indicated as IS.

Gate

· The Gate terminal in a Field Effect Transistor plays a key role in the function of FET by controlling the current through the channel.

· By applying an external voltage at Gate terminal, the current through it can be controlled.

· Gate is a combination of two terminals connected internally that are heavily doped.

· The channel conductivity is said to be modulated by the Gate terminal.

· This is analogous to the base terminal in a Bipolar Junction Transistor.

· The Gate terminal can be designated as G.

· The current entering the channel at Gate terminal is indicated as IG.

Drain

· The Drain terminal in a Field Effect Transistor is the one through which the carriers leave the channel.

· This is analogous to the collector terminal in a Bipolar Junction Transistor.

· The Drain to Source voltage is designated as VDS.

· The Drain terminal can be designated as D.

· The current leaving the channel at Drain terminal is indicated as I_D.

Types of FET

There are two main types of FETS. They are JFET and MOSFET. The following figure gives further classification of FETs.