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Field Effect TransistorField Effect Transistor

(FET)(FET)

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Transistors

BJT FET

NPN PNP

IGBT

MOSFET JFET TRIAC Thyristor

Types of TransistorsTypes of Transistors

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SimilaritiesSimilarities:: Amplifiers

Switching devices

Impedance matching circuits

Differences:Differences:

FETs are voltage controlled devices. BJTs are current

controlled devices.

FETs have a higher input impedance. BJTs have higher

gains.

FETs are less sensitive to temperature variations and are

more easily integrated on ICs.

FETs are generally more static sensitive than BJTs.

FETsFETs vsvs BJTsBJTs

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JFETJFET:: Junction FET

MOSFETMOSFET:: MetalOxideSemiconductor FET

D-MOSFET: Depletion MOSFET

E-MOSFET: Enhancement MOSFET

FETsFETs TypesTypes

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JFET Advantages & UsesJFET Advantages & Uses

AdvantagesAdvantages low power

high gate impedance

low S/Dresistance

UsesUses amplifier

analog switch

DesignDesign gate==base

source==emitter

drain ==collector

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JFET SymbolsJFET Symbols

N-Channel JFET Symbol P-Channel JFET Symbol

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JFETJFET ConstructionConstruction

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There are two types of JFETs

n-channel

p-channel

JFET ConstructionJFET Construction

The n-channel is more widely used.

There are three terminals:

Drain (D) and Source (S) are connected to the n-channel

Gate (G) is connected to thep-type material

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JFET Operation: The Basic IdeaJFET Operation: The Basic IdeaJFET operation can be compared to a waterspigot.

The source of waterpressure is the accumulation

of electrons atthe negative pole ofthe drain-source

voltage.

The drain of water is the electron deficiency (or

holes) atthe positive pole ofthe applied voltage.

The control of flow of water is the gate voltage thatcontrols the width ofthe n-channel and, therefore,

the flow of charges from source to drain.

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There are three basic operating conditions for aJFET:

VGS = 0, VDS increasing to some positive value

VGS < 0, VDS at some positive value

Voltage-controlled resistor

JJFET Operating CharacteristicsFET Operating Characteristics

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JFET Operating CharacteristicsJFET Operating Characteristics

(V(VGSGS

= 0= 0 V)V)

1. The depletion region between p-gate and

n-channel increases as electrons fromn-channel combine with holes from p-gate.

2. Increasing the depletion region, decreases

the size of the n-channel which increases

the resistance of the n-channel.

3. Even though the n-channel resistance is

increasing, the current (ID) from source to

drain through the n-channel is increasing.

This is because VDS is increasing.

Three things happen when VGS = 0 and VDS is increased

from 0 to a more positive Voltage

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(V(VGSGS

= 0= 0 V)V)If VGS = 0 and VDS is furtherincreased to a more positive

voltage, then the depletion zone

gets so large that it pinches off the

n-channel.

This suggests that the current in

the n-channel (ID) would drop to

0A, but it does just the opposite as VDS increases, so does

ID. But as this happens the

depletion region tightens and the

resistance increases so the

current goes back to IDSS.

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((SaturationSaturation))

Any further increase in VDS

does not produce any increasein ID. VDS at pinch-off is denoted

as Vp.

ID is at saturation or maximum.

It is referred to as IDSS.

The ohmic value of the channel

is maximum.

At the pinch-off point:

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As VGS becomes more negative,

the depletion region increases.

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As VAs VGSGS becomes more negative:becomes more negative:

The JFET experiences

pinch-off at a lower

voltage (VP).

ID decreases (ID < IDSS)

even though VDS isincreased.

Eventually ID reaches 0A.

VGS at this point is called

VGS(off)..

VGS(off)VGS(off)

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Also note that at high levels ofAlso note that at high levels of

VVDSDS the JFET reaches athe JFET reaches a

breakdown situation. Ibreakdown situation. IDD

increases uncontrollably ifincreases uncontrollably if

VVDSDS>> VVDSmaxDSmax

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Voltage-Controlled ResistorThe region to the left of the

pinch-off point is called the

ohmic region.

The JFET can be used

as a variable resistor,

where VGS controls the

drain-source resistance

(rd). As VGS becomes

more negative, the

resistance (rd)

increases.

VGS(off)VGS(off)

o

d2

GS

P

rr =

V1 -

V

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pp--ChannelJFETSChannelJFETS

Thep-channelJFET

behaves the same as

the n-channelJFET,

except the voltage

polarities and current

directions are reversed.

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JFET Transfer CharacteristicsJFET Transfer Characteristics

The transfer characteristic of input-to-output is not asstraightforward in a JFET as it is in a BJT.

In a BJT, indicates the relationship between IB (input) and IC

(output).

In a JFET, the relationship of VGS (input) and ID (output) is a

little more complicated:

2GS

D DSS

P

VI = I 1 -

V

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JFET Transfer CurveJFET Transfer Curve

This graph shows the value of ID for a given value of VGS. And

effect of VDS on ID for that VGS.

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Plotting the JFET Transfer CurvePlotting the JFET Transfer Curve

Using IDSS and VGS(off) values found in a specification sheet, the transfercurve can be plotted according to these three steps:

Step 1:

Solve for VGS = 0V

Step 2

Solve for VGS = Vp ( aka VGS(off) )

Step 3:

Solve for 0V uVGSu Vp in 1V

increments for VGS

2

I I 1 -

2

I I 1 -

2

I I 1 -

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JFET Specifications SheetJFET Specifications SheetElectrical Characteristics

(TA = 25 Deg Centigrade unless otherwise noted)

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JFET Specifications SheetJFET Specifications Sheet

Maximum RatingsMaximum Ratings

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Case and Terminal IdentificationCase and Terminal Identification

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MOSFETsMOSFETs

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MOSFETsMOSFETs

MOSFETs have characteristics similar to JFETs and additionalcharacteristics that make them very useful

There are 2 types ofMOSFETs:

Depletion modeMOSFET (D-MOSFET)

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

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

EnhancementModeMOSFET (E-MOSFET) Operates in Enhancement mode

IDSS = 0 until VGS > VT (threshold voltage)

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Depletion ModeDepletion Mode

TypeType MOSFETsMOSFETs

(D Type(D Type--MOSFE

T)MOSFE

T)

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The Drain (D) and Source (S) leads connectto the to n-doped regions

These N-doped regions are connected via an n-channel

This n-channel is connected to the Gate (G) via a thin insulating layerofSiO2

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

DepletionMode MOSFETConstructionDepletionMode MOSFETConstruction

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DepletionMode MOSFETBasic OperationDepletionMode MOSFETBasic Operation

A D-MOSFET may be biased to operate in two modes:

the Depletion mode or the Enhancement mode

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The transfer characteristics are similar to the JFET. So In DepletionMode operation:

When VGS = 0V, ID = IDSS

When VGS < 0V, ID < IDSS

The formula used to plot the Transfer Curve, is:

2

GS

D DSS

P

VI = I 1 -

V

DD--MOSFETDepletion Mode OperationMOSFETDepletion Mode Operation

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In this mode, the transistoroperates with VGS > 0V, and ID increases above IDSSShockleys equation, the formula used to plotthe TransferCurve, still applies but

VGS is positive:

DD--MOSFETEnhancementMode OperationMOSFETEnhancementMode Operation

2

D DSS

P

I I 1 -

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The p-channel Depletion mode MOSFET is similarto the n-channel exceptthat

the voltage polarities and current directions are reversed

pp--Channel DChannel D--MOSFETEnhancementModeMOSFETEnhancementMode

OperationOperation

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DD--MOSFETEnhancementSymbolsMOSFETEnhancementSymbols

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SpecificationSheetSpecificationSheet

Maximum Ratings

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Electrical Characteristics

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Enhancement ModeEnhancement Mode

TypeType MOSFETsMOSFETs(EType(EType--MOSFET)MOSFET)

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The Drain (D) and Source (S)

connect to the to n-doped regions.

These n-dopedregions are not

connected via an n-channel

The Gate (G) connects to thep-doped

substrate via a thin insulating layer of

SiO2.

There is no channel

The n-doped material lies on a p-

dopedsubstrate that may have an

additional terminal connection

called the Substrate (SS).

EnhancementMode MOSFETConstructionEnhancementMode MOSFETConstruction

(n(n Channel)Channel)

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EnhancementMode MOSFETBasic OperationEnhancementMode MOSFETBasic Operation

The enhancement type MOSFET operates only in the enhancement mode

VGS is always positive

As VGS is kept constant and VDS is increased, then ID saturates (IDSS)

and the saturation level, VDSsat is reached

As VGS increases, ID increases

VDSsat

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EE--Type MOSFETTransfer CurveType MOSFETTransfer Curve

To determine ID given VGS:

k= a constant, can be determined by using values at a specificpoint and the formula:

Where: VT = threshold voltage or voltage at which the MOSFET turns on

2)(TGSD

VVkI !

2

)(

)(

)(TONGS

OND

VV

Ik

!

VDSsat can be calculated by:

VDsat = VGS VT

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pp ChannelEChannelE--Type MOSFETType MOSFET

Thep-channel enhancement-type MOSFETis

similar to the n-channel, except that the

voltage polarities and current directions are

reversed.

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MOSFET SymbolsMOSFET Symbols

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Maximum Ratings

*Transient potentials of 75 Volts will not cause gate-oxide failure.

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Electrical Characteristics

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Handling MOSFETsHandling MOSFETsMOSFETs are very sensitive to static electricity. Because of

the very thin SiO2 layer between the external terminals and

the layers of the device, any small electrical discharge cancreate an unwanted conduction.

Always transport in a static sensitive bag.

Always wear a static strap when handling

MOSFETS.

Apply voltage limiting devices between the gate

and source, such as back-to-back Zeners to limit

any transient voltage.

ProtectionProtection

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Summary TableSummary Table

pFC

MR

j

i

)101(:

100

;"

JFETJFET

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pj

i

)101(:

1010;"

Summary TableSummary Table

EE--Type MOSFETType MOSFET

Documents

Final FET1