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Prepared by:

Engr. IRA C. VALENZUELA

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INTRODUCTION

Field Effect Transistors are preferred for weaksignal work.

They are also preferred in circuits and systemrequiring high impedance

FETs are fabricated onto a silicon integrated

circuit (IC) chipsVariations of FET technology are based ondifferent ways of generating the electric field.

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HISTORY

October 22, 1925The first patent for the field effect transistor principlewas filed in Canada by Austrian-Hungarian physicist

Julius Edgar Lilienfeld1934German physicist Dr. Oskar Heil patented another fieldeffect transistor

Legal papers from the Bell Labs patent show thatWilliam Shockley and a co-worker at Bell Labs, GeraldPearson, had built operational versions from Lilienfelds

patents

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DEFINITION

The field-effect transistor (FET) is a three-terminal device

The FET is a unipolar device depending solelyon either electron (n-channel) or hole (p-

channel) conduction.

FET transistor is a voltage-controlled device.

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FET vs BJT

1. The FET has extremely high input resistancewith about 100 M typically (BJT input

resistance typically 2 k).

2. The FET has no offset value when used as aswitch.

3. The FET is relatively immune to radiation butthe BJT is very sensitive.

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FET vs BJT

4. The FET is less noisy than BJT.

5. The FET can be operated to provide greaterthermal stability than BJT.

6. FET is smaller than BJT.

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FET vs BJT

7. FET has smaller gain bandwidth than BJT.

8. FET has greater susceptibility to damage inhandling.

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FET vs BJT

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Types of FET

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Drain DrainChannel

Gate Gate

Source Source

P - CHANNEL N - CHANNEL

JFET Construction

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Drain

Gate

Source

Drain

Gate

Source

N - CHANNEL P - CHANNEL

JFET Schematic Symbol

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Depletion and Pinch-off

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Depletion and Pinch-off

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Biasing FET

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JFET biased for conduction

P P

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Greater VGG

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Less VGG

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Drain-Source Characteristic Curve

It is a plot of drain current versus the drain-source voltage.

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Drain-Source Characteristic Curve

It is a plot of drain current versus the drain-source voltage.

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JFET Transfer Characteristic Curve

It is a plot of drain current as a function of gate-source voltage.

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Transconductance

It is also called dynamic mutual conductance

If the gate-source voltage changes by a smallamount dVGS then the drain current will alsochange by a certain increment dID.

The transconductance is the ratio dID/ dVGS.

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Transconductance

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Transconductance

0

DSvGS

D

m V

Ig

P

GS

mom V

V

gg 1

gmo = the maximum AC gain parameter of the JFET

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JFET Parameters

2

1

P

GS

DSSD

V

VII

P

GS

P

DSS

m

V

V

V

Ig 1

2

ID = drain current

IDSS = drain-sourcesaturation currentVGS = gate-sourcevoltage

VP = pinch-off voltagegm = transconductance

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Sample Problems

1. Determine the drain current of an n-channel JFEThaving a pinch-off voltage VP = - 4 V and the drain-

source saturation current IDSS = 12 mA at VGS = 0and VGS = - 3 V.

12 mA, 0.75 mA

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Sample Problems

2. Calculate the transconductance, gm, of a JFET

with IDSS = 12 mA and VP = - 4 V at bias pointVGS = -1.5 V.

3.75 mS

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Sample Problems

3. What is the value of IDSS with gmo = 4.5 mSand VP = - 3 V?

6.75 mA

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Sample Problems

4. What is the value of VP of a p-channel JFEThaving IDSS = 12 mA and gmo = 6500 S?

3.69 V

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Sample Problems

5. Determine the value of gmo for a p-channel

JFET having VP = 3.8 V and IDSS = 6.8 mA.

3.58 mS

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Sample Problems

6. A p-channel JFET with IDSS = 13.5 mA, VP = 5V is operated at ID = 9.5 mA. What is the value ofgm at this operating point?

4.525 mS

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Sample Problems

7. What is the maximum value of

transconductance of a JFET (VP = - 4 V) if thetransconductance is 4500 S when operated atVGS = - 1 V?

6 mS

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Important Relationships

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JFET Biasing

Fixed Bias Configuration

Self-Bias Configuration

Voltage Divider Biasing

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Fixed Bias

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Fixed Bias: Biasing equations

DDDDDS

2

P

GSDSSD

GGGS

RIVV

V

V1II

VV

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Fixed Bias Configuration

Vgs = -2 V

Id = 5.625 mA

Vds = 4.75 V

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Self-Bias

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Self-Bias: Biasing equations

)RR(IVV

V

V1II

RIV

SDDDDDS

2

)off(GS

GSDSSD

SDGS

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Self-Bias Configuration

Vgs = - 2.6 V

Id = 2.6 mAVds = 8.82 V

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Voltage-Divider Bias

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Voltage-Divider Bias: Biasing equations

)RR(IVV

R

VVI

RR

R

VV

SDDDDDS

S

GSGD

21

2

DDG

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Voltage Divider Biasing

Vgs = - 1.8 VId = 2.4 mA

Vd = 10.24 V

Vs = 3.6 V

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MOSFET

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MOSFET

The acronym MOSFET stands for metal-oxide-semiconductor field-effect transistor.

MOSFETs are further broken down into depletiontype and enhancement type.

The insulating layer between the gate and channelhas resulted in another name for the device:insulated gate FET or IGFET

MOSFET h h t i ti i il t JFET

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45

MOSFETs have characteristics similar to JFETs

and additional characteristics that make then very

useful

There are 2 types of MOSFETs:

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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Schematic Symbol

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MOSFET Terminal Characteristics

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The main problem

The trouble with MOSFETs is that they can beeasily damaged by static electric discharges.

If a static discharge occurs through the dielectric of

a MOS device, the component will be destroyedpermanently.

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Depletion MOSFET

Drain

Gate

Source

Channel

SiO2

p

n

n

Basic structure of

n-channel D-MOSFET

n-channel D-MOSFET is

usually operated in the

depletion mode with VGS

< 0 and in theenhancement mode with

VGS > 0.

p-channel D-MOSFETuses the opposite

voltage polarity

G

D

S

Symbol

D MOSFET S b l

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50

D-MOSFET Symbols

D MOSFET Depletion Mode Operation

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51

D-MOSFET Depletion Mode Operation

The transfer characteristics are similar to the JFET

In Depletion Mode operation:When VGS = 0V, ID = IDSS

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

D MOSFET E h M d O i

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52

D-MOSFET Enhancement Mode Operation

Enhancement Mode operation

In this mode, the transistor operates with VGS > 0V, and ID increases above IDSSShockleys equation, the formula used to plot the Transfer Curve, still applies but

VGS is positive:

2

GS

D DSS

P

VI = I 1 -

V

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Basic Operation

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p-Channel Depletion-Type MOSFET

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Symbols

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DMOSFET

Vgs = - 0.8 V

Id = 3.1 mA

Vds = 10.1 V

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ENHANCEMENT-TYPEMOSFET

The transfer curve is not defined by Shockleysequation.

The drain current is now cut off until the gate-to-source voltage reaches a specific magnitude.

Current control in an n-channel device is noweffected by a positive gate-to-source voltage.

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ENHANCEMENT-TYPEMOSFET

The construction of anenhancement-typeMOSFET is quitesimilar to that of thedepletion-typeMOSFET, except for

the absence of achannel between thedrain and sourceterminals.

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Basic Operation

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SYMBOLS

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EMOSFET Parameters

2THGSGSD

VVkI

THGSGSm VVkg 2

k = 0.3 mA/V2

VGS(TH) = threshold voltage

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Advantages of MOSFET

draws no gate current at all

draws no leakage current

the input resistance of the device

is essentially infinite.

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Disadvantages ofMOSFET

that thin layer of glass cantwithstand much voltage

the static charge can destroy thedevice

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Sample Problem

A depletion MOSFET with IDSS

= 12 mA, VP

=-4V is operated at VGS = - 0.5 V. What is the valueof the transconductance at this operating point?

5. 25 mS

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Sample Problem

What is the value of threshold voltage for an n-channel enhancement MOSFET that operates atID = 4.8 mA when biased at 7 V?

3 V

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Sample Problem

An enhancement MOSFET having threshold of3.5 V is operated at VGS = 5 V. What currentresults?

675 A

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Sample Problem

Determine the value of circuit transconductancefor an n-channel enhancement MOSFET havingVGS(TH) = 2.8 V when operated at 6 V.

1.92 mS

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Sample Problem

An enhancement MOSFET operated at VGS

= 7.5V has transconductance of 2.5 mS. What is thevalue of a device threshold voltage?

3.33 V

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Sample Problem

Measurements taken with E-MOSFET indicatethat when VGS = 4 V, ID = 8 mA and when VGS =6 V, ID = 32 mA. Determine the value of k.

2 mA/V2

Th F ll i F t f EMOSFET th t

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The Following Features of EMOSFETs thatare Common with DMOSFETs:

Charge-carrier flow from the source to drain.

The type of semiconductor material used for thechannel is opposite the type of material used forthe substrate.

The arrow part of the schematic symbol indicatesthe type of material that is used for the substrate.

EMOSFET

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EMOSFET

Vgs = 6.4 V

Id = 2.75 mA

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VMOS

Vertical Metal-Oxide-Silicon FET

Compared with commercially available planarMOSFETs, VMOS FETs have reduced channelresistance levels and higher current and powerratings.

VMOS FETs have a positive temperaturecoefficient that will combat the possibility ofthermal runaway.

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VMOS

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CMOS

Complementary MOSFET

It has extensive applications in computer logicdesign.

The relatively high input impedance, fast

switching speeds, and lower operating powerlevels of the CMOS configuration have resultedin a whole new discipline referred to as CMOSlogic design.

Si l MOS lifi

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78

Characteristic parameters

Av Ai Zi Zo

Three configurations

Common-source configuration

Common-drain configuration

Common-gate configuration

Single-stage MOS amplifier

B i t t f th i it

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79

Basic structure of the

circuit used to realizesingle-stage discrete-

circuit MOS amplifier

configurations.

Basic structure of the circuit

Th lifi

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SJTU J. Chen

80

The simplest common-sourceamplifier biased with constant-

current source.

CC1 And CC2 are coupling

capacitors.CS is the bypass capacitor.

The common-source amplifier

Ch t i ti f CS lifi

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81

Input resistance

Voltage gain

Overall voltage gain

Output resistance

Gin RR

)////( LDomv RRrgA

)////( oLDmsigG

Gv rRRg

RRRG

Doout RrR //

Characteristics of CS amplifier

Summary of CS amplifier Very high input resistance

Moderately high voltage gain

Relatively high output resistance

Th C G t lifi

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83

Biasing with constant

current source I

Input signalvsig is

applied to the source

Output is taken at thedrain

Gate is signal grounded

CC1 and CC2 are coupling

capacitors

The Common-Gate amplifier

Th CG lifi

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84

The CG amplifier

A small-signal equivalent

circuit

T model is used in

preference to the model

Ro is neglected

The CG amplifier fed with a current signal input

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SJTU J. Chen

85

The CG amplifier fed with a current-signal input

Voltage gain


)//( LDmv RRgA

sigm

LDm

v Rg

RRg

G 1

)//(

S f CG lifi

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86

Noninverting amplifier

Low input resistance

Relatively high output resistanceCurrent follower

Superior high-frequency performance

Summary of CG amplifier

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88

The CD or source-follower amplifier

Small-signal equivalent-

circuit model

T model makes analysissimpler

Drain is signal grounded


11

//

//

m

Lo

Lo

sigG

Gv

gRr

Rr

RR

RG

Summary of CD or source follow amplifier

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89

Very high input resistance

Voltage gain is less than but close to unity

Relatively low output resistance

Voltage buffer amplifier

Power amplifier

Summary of CD or source-follow amplifier

Other FET Applications

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A voltage controlled attenuator

for small drain-to-source

voltages FETs resemble

voltage-controlled resistors

the gate voltage VG is used

to control this resistance and

hence the gain of the potentialdivider

used, for example, in automatic

gain control in radio receivers

O h A l

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A FET as an analogue switch



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A FET as a logical switch


fet (2014).pdf

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