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Field Effect Transistor

Basic information

FET

<> FETs are majority-charge-carrier devices. The device consists of an active channel through which majority charge carriers, electrons or holes, flow from the source to the drain. Source and drain terminal conductors are connected to semiconductor through ohmic contacts. The conductivity of the channel is a function of potential applied to the gate. The FET's three terminals are:

• Source (S), through which the majority carriers enter the channel. Conventional current entering the channel at S is designated by IS.
• Drain (D), through which the majority carriers leave the channel. Conventional current entering the channel at D is designated by ID. Drain to Source voltage is VDS.
• Gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control ID.

ID=IDSS(1-VGS/VP)^2

Advantages of FET

The main advantage of the FET is its high input resistance, on the order of 100M ohms or more. Thus, it is a voltage-controlled device, and shows a high degree of isolation between input and output. It is a unipolar device, depending only upon majority current flow. It is less noisy and is thus found in FM tuners for quiet reception. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper. It typically has better thermal stability than a BJT.

Disadvantages of FET

It has relatively low gain-bandwidth product compared to a BJT. The MOSFET has a drawback of being very susceptible to overload voltages, thus requiring special handling during installation.

MOSFET

Depletion type MOSFET

There are two n-type island on p-type substrate. Between these two n regions there is a n-channel. The two n-regions form Source and Drain terminals. Gate terminal is to insulated layer of SiO2. There is conduction without any Gate voltage.

D MOSFET

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Enhancement type MOSFET

Similar to Depletion type , but without n-channel. Hence for conduction there is requirement of some positive Gate voltage which attracts electrons from p-region which conducts from Source to Drain.

E MOSFET

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Applications

• Phase-Shift Oscillator
• Voltage variable resistor (in ohmic region)
• High input resistance solid-state DC voltmeter

FET amplifier

INTRODUCTION

The main advantage of FET is used for amplification is that it has very high input impedance. Also it shows low output impedance. These two are main conditions for amplifier.

In detail

The transconductance is given by,

gm=Id/Vgs

On rearranging we get,

Id=gm*Vgs ...(1)

Thus, from this equation we can tell that output current Id is product of input voltage Vgs and the transconductance gm.

Equivalent circuit

A FET equivalent circuit is given by equation number (1). The internal resistance r'gs , between gate and source and a current source given by equation (1) appears between drain and source. r'ds is internal resistance between drain and source. As r'gs is very high, it is taken to be infinite and r'ds is neglected.

Voltage Gain

For ideal FET equivalent circuit, voltage gain is given by,

Av=Vds/Vgs

From the equivalent circuit,

Vds=Id*Rd

and from the definition of transconductance,

Vgs=Id/gm

Thus we get,

Av=gm*Rd ...(2)

Types of FET amplifiers

There are three types of FET amplifiers depending upon the common terminal used as input and output similar to BJT amplifier.

1. Common Gate Amplifier

In this the gate terminal is common to both input and output.

See also : Common gate

2. Common Source Amplifier

In this the source terminal is common to both input and output.

See also : Common source

3. Common Drain Amplifier

In this the drain terminal is common to both input and output. It is also known as 'Source Follower'.

See Also : Common drain