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Lecture on

Field Effect Transistor (FET)

by:-Uttampreet Singh

Lecturer-Electrical Engg.Govt. Polytechnic College, G.T.B.garh, Moga

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What is FET?

FET is uni-polar device i.e. operation depends on only one type of charge carriers (h or e) . It is a Voltage controlled Device (gate voltage controls drain current)

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1. Very high input impedance (109-1012 )

2. Source and drain are interchangeable

3. Low Voltage Low Current Operation is possible (Low-power consumption)

4. Less Noisy 5. No minority carrier storage (Turn off is faster) 6. Very small in size, occupies very small space

in ICs

ADVANTAGES OF FET

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Current Controlled vs Voltage Controlled Devices

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Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

n-Channel JFET

p-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FETFETFET » JFET

MOSFET (IGFET)

FET » JFET

MOSFET (IGFET)

FET

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JFET ConstructionThere are two types of JFET’s: n-channel and p-channel.

The n-channel is more widely used.

There are three terminals: Drain (D) and Source (S) are connected to n-channelGate (G) is connected to the p-type material

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The nonconductive depletion region becomes thicker with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

N-Channel JFET Operation

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Gate

Drain

Source

SYMBOLS

n-channel JFET

Gate

Drain

Source

p-channel JFET

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CHARATERISTICS

At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS.

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ID IDSS

As VGS becomes more negative:• the JFET will pinch-off at a lower voltage (Vp).• ID decreases (ID < IDSS) even though VDS is increased.• Eventually ID will reach 0A. VGS at this point is called Vp or VGS(off).• Also note that at high levels of VDS the JFET reaches a breakdown situation. ID will increases uncontrollably if VDS > VDSmax.

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Transfer Characteristics

The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT

In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):

As a result, FET’s are often referred to a square law devices

2GS

D DSSP

V I = I 1 -

V

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Transfer (Transconductance) Curve

From this graph it is easy to determine the value of ID for a given value of VGS

It is also possible to determine IDSS and VP by looking at the knee where VGS is 0

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Case Construction and Terminal Identification

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p-Channel JFET

p-Channel JFET operates in a similar manner as the n-channel JFET except the voltage polarities and current directions are reversed

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P-Channel JFET Characteristics

As VGS increases more positively• the depletion zone increases• ID decreases (ID < IDSS)• eventually ID = 0AAlso note that at high levels of VDS the JFET reaches a breakdown situation. ID increases

uncontrollably if VDS > VDSmax.

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MOSFET(Metal Oxide Semiconductor FET)

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MOSFET

There are two types of MOSFET’s:• Depletion mode MOSFET (D-MOSFET)

• Operates in Depletion mode the same way as a JFET when VGS 0

• Operates in Enhancement mode like E-MOSFET when VGS > 0

• Enhancement Mode MOSFET (E-MOSFET)• Operates in Enhancement mode• IDSS = 0 until VGS > VT (threshold voltage)

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Depletion Mode MOSFET Construction

The Drain (D) and Source (S) leads connect to the to n-doped regionsThese N-doped regions are connected via an n-channelThis n-channel is connected to the Gate (G) via a thin insulating layer of SiO2

The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS

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D-MOSFET Symbols

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Basic Operation

A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode

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p-Channel Depletion Mode MOSFET

The p-channel Depletion mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed

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Enhancement ModeMOSFET’s

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Enhancement Mode MOSFET Construction

The Drain (D) and Source (S) connect to the to n-doped regionsThese n-doped regions are not connected via an n-channel without an external voltageThe Gate (G) connects to the p-doped substrate via a thin insulating layer of SiO2

The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS

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E-MOSFET Symbols

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Basic OperationThe Enhancement mode MOSFET only operates in the enhancement mode.

VGS is always positiveIDSS = 0 when VGS < VT

As VGS increases above VT, ID increasesIf VGS is kept constant and VDS is increased, then ID saturates (IDSS)The saturation level, VDSsat is reached.

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p-Channel Enhancement Mode MOSFETs

The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.

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Summary Table

JFET D-MOSFET E-MOSFET

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