JFET and MOSFET Types and Operation: A Comprehensive Guide, Lecture notes of Electronics

Download JFET and MOSFET Types and Operation: A Comprehensive Guide and more Lecture notes Electronics in PDF only on Docsity!

Chapter 6: Field-Effect Transistors

FETs vs BJTsFETs vs BJTs

Similarities:Similarities:

FETs

vs. BJTs

FETs vs. BJTs

Similarities:Similarities:

• Amplifiers• Switching devices• Impedance matching circuitsImpedance matching circuits

Differences:Differences:

• FETs are voltage controlled devices

BJTs are current controlled

FETs are voltage controlled devices. BJTs are current controlleddevices.• FETs have a higher input impedance. BJTs have higher gains.• FETs are less sensitive to temperature variations and are more easilyFETs are less sensitive to temperature variations and are more easilyintegrated on ICs.• FETs are generally more static sensitive than BJTs.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

22

JFET ConstructionJFET Construction

There are two types of JFETs

• • nn - -channel channel
  • • pp - -channelchannel Th

h^

l i^

id l

d

The n-channel is more widely used.^ There are three terminals:There are three terminals:

• •DrainDrain (D) and Source

Source (S) are connected to the

n -channel

• •GateGate (G) is connected to the

p -type material

• •GateGate (G) is connected to the

p -type material

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

44

JFET Operation: The Basic IdeaJFET Operation: The Basic Idea

JFET operation can be compared to a water spigot.

The sourceThe source

of water pressure is the

accumulation of electrons at the

i^

l^

f h

d i

negative pole of the drain-sourcevoltage.Th^

d^

i Th^

d^

i^

f^

t^

i^ th

l

t

The drainThe drain of water is the electrondeficiency (or holes) at the positivepole of the applied voltage. The controlThe control

of flow of water is the

gate voltage that controls the widthof the n-channel and therefore theof the n-channel and, therefore, theflow of charges from source todrain.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

55

JFET Operating Characteristics: VJFET Operating Characteristics: V

GSGS

= 0 V= 0 V

Three things happen when V

= 0 and VGS

is increased from 0 to a more positiveDS

voltage

•^

The depletion region between p-gateand n-channel increases as electronsfrom n channel combine with holes g

from n-channel combine with holesfrom p-gate.

-^

Increasing the depletion regionIncreasing

the depletion region,

decreases the size of the n-channelwhich increases the resistance of then-channel.n channel.

-^

Even though the n-channel resistanceis increasing, the current (I

) fromD

g,^

(^ D

source to drain through the n-channel is increasing. This is becauseVDS

is increasing.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

77

JFET Operating Characteristics:JFET Operating Characteristics:

Pinch OffPinch Off

p^

g

p^

g

If V

= 0 and VGS

is further increased toDS

a more positive voltage, then thed^

l^ i

l^

h^

i

depletion zone gets so large that itpinches offpinches off the n-channel.Thi

t^ th t th

t i^

th

This suggests that the current in the n-channel (I

) would drop to 0A, but it doesD

just the opposite–as V

increases, so doesDS^

II.D^

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

88

JFET Operating CharacteristicsJFET Operating Characteristics

p^

g

p^

g

A^

V^

b^

ti^

th

As V

becomes more negative, theGS depletion region increases.

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1010

JFET Operating CharacteristicsJFET Operating Characteristics

As V

becomes more negative:GS

p^

g

p^

g

•^

The JFET experiencespinch-off at a lower voltage (VP

•^

Idecreases (ID^

< ID

DSS ) even

th^

h V

i

i^

d

though V

is increased.DS

•^

Eventually I

reaches 0 A.D

V^

at this point is called V VGS

at this point is called V

p

or V

GS(off)

A^

i^

f^

i^

i

Also note that at high levels of V

the JFET reaches a breakdown situation. IDS

D

increases uncontrollably if V

> VDS^

DSmax

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1111

pp--Channel JFETS

Channel JFETS

pp

The

p -channel JFET behaves the The

p -channel JFET behaves the same as the

n -channel JFET,

except the voltage polarities andcurrent directions are reversedcurrent directions are reversed

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1313

pp--Channel JFET Characteristics

Channel JFET Characteristics

pp Channel JFET Characteristics

Channel JFET Characteristics

A^

V^

i^

iti^

l

As V

increases more positivelyGS (^) • The depletion zoneincreasesincreases

-^

Idecreases (ID^

< ID

DSS

•^

Eventually I

= 0 AD

Also note that at high levels of V

the JFET reaches a breakdown situation: IDS^

D

increases uncontrollably if V

> VDS

DSmax

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1414

JFETJFET

Transfer CharacteristicsTransfer Characteristics

JFETJFET

Transfer

Characteristics

Transfer Characteristics

The transfer characteristic of input-to-output is not as straightforward ina JFET as it is in a BJT.In a BJT,

β^ indicates the relationship between I

(input) and IB^

(output).C

In a JFET the relationship of V

(input) and IGS

(output) is a little moreD

In a JFET, the relationship of V

(input) and IGS

(output) is a little moreD

complicated:

2 V V 1 DSS D

GS P

I

I^

⎞⎟ ⎟⎟⎠ ⎛⎜−⎜⎜⎝

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1616

JFET Transfer CurveJFET Transfer Curve

This graph shows thevalue of I

for aD^

given value of Vgiven value of V

.GS

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1717

JFET Specifications SheetJFET Specifications Sheet

Electrical CharacteristicsElectrical Characteristics

Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1919

JFET Specifications SheetJFET Specifications Sheet

Maximum RatingsMaximum Ratings

more…more… Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

2020