ELEKTRONIKOS ĮTAISAI 2009 - VGTU · A field-effect transistor (FET) is a three-terminal device in...

ELEKTRONIKOS ĮTAISAI 2009

VGTU EF ESK [email protected]

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Field Effect Transistors

Lauko tranzistoriuose (angl. FET – field effect transistor) srovę kuria specialiai sudaryto

kanalo pagrindiniai krūvininkai. Kadangi srovę lemia vieno ženklo krūvininkai, šie

tranzistoriai dar vadinami vienpoliais tranzistoriais. Kanalo laidumą ir juo tekančią srovę

valdo statmenas srovės krypčiai elektrinis laukas.

Lauko tranzistoriaus n arba p kanalo (angl. – channel) gale sudaromi du elektrodai.

Elektrodas, per kurį į kanalą patenka pagrindiniai krūvininkai, vadinamas ištaka (source).

Elektrodas, per kurį pagrindiniai krūvininkai išteka, vadinamas santaka (drain). Kanale

tekančią srovę valdo trečiojo tranzistoriaus elektrodo – užtūros (gate) – įtampa.

Pagal užtūros tipą lauko tranzistoriai skirstomi į lauko tranzistorius su valdančiosiomis pn

sandūromis (sandūrinius lauko tranzistorius) ir lauko tranzistorius su izoliuotąja užtūra.

A field-effect transistor (FET) is a three-terminal device in which current flows

through a narrow conducting channel between two electrodes called source and

drain. The current is modulated by the electric field caused by voltage applied at

the third electrode called gate. Current flow along the channel is almost entirely

due to the motion of majority carriers. So, the FET is a unipolar device and

there are two types of FETs: n-channel devices and p-channel devices.



2

History

In 1926 Julius Edgar Lilienfeld applied for three patents. The first two, from 1926

and 1928, describe what we now call a field-effect transistor (FET) structure. The

first patent (J.E. Lilienfeld, Method and apparatus for controlling electric currents,

US Patent 1,745,175, application filed October 8, 1926, granted January 18,

1930) gives a MESFET or metal/semiconductor FET. The second patent (J.E.

Lilienfeld, Device for controlling electric current, US Patent 1,900,018 application

filed March 28, 1928, patented March 7, 1933) is derived from the first, and gives

a depletion mode MOSFET.

In 1960 Bell scientist John Atalla developed a new design (MOSFET) based on

William Shockley's original field-effect theories.

http://www.electro.patent-invent.com/electricity/inventions/fet.html#History



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Types of FETs

The FET is simpler in concept than the bipolar transistor and can be constructed from a

wide range of materials. The channel region of any FET is either doped to produce an N-

type semiconductor, giving an "N-channel" device, or with a P-type to give a "P-channel"

device. The doping determines the polarity of gate operation. The different types of field-

effect transistors can be distinguished by the method of insulation between channel and

gate:

The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an insulator

(typically SiO2).

The JFET (Junction Field-Effect Transistor) uses a p-n junction as the gate.

The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n-junction

of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor

materials.

Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High

Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully

depleted wide-band-gap material forms the isolation.

The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure

formed by graded doping of the active region.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or

other amorphous semiconductors in thin-film transistors or organic field effect transistors

that are based on organic semiconductors and often apply organic gate insulators and

electrodes.



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The FET is simpler in concept than the bipolar transistor and can be constructed from a

wide range of materials. The different types of field-effect transistors can be distinguished

by the method of isolation between channel and gate:

The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an isolator

(typically SiO2).

Power MOSFETs become less conductive with increasing temperature and can

therefore be thought of as n-channel devices by default. Silicon devices that use

electrons, rather than holes, as the majority carriers are slightly faster and can carry

more current than their P-type counterparts. The same is true in GaAs devices.

The JFET (Junction Field-Effect Transistor) uses a p-n junction as the gate.

The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n-junction of

the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.

Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High

Electron Mobility Transistor), also named an HFET (heterostructure FET). The fully

depleted wide-band-gap material forms the isolation.

TFTs (thin-film transistor) use amorphous silicon, polycrystalline silicon or other amorphous

semiconductors as body material.

A subgroup of TFTs are organic field effect transistors that are based on organic

semiconductors and often apply organic gate insulators and electrodes.

The channel region of any FET is either doped to produce n-type semiconductor, giving an

"N-channel" device, or with p-type to give a "P-channel" device. The doping determines the

polarity of gate operation.

Types of FETs



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(FETs)

Junction FETs (JFETs) LT su valdančiosiomis

pn sandūromis –

sandūriniai LT

IG FETs, MISFETs,

MOSFETsLT su izoliuotąja užtūra

– MDP LT, MOP LT

E-MOSFETsLT su indukuotuoju

kanalu - praturtintosios

veikos LT

DE-MOSFETsLT su įterptuoju kanalu

– nuskurdintosios-praturtintosios

veikos LT

GAAS FETs, MESFETsMetalo-puslaidininkio

(MP) LT HEMTsHeterostruktūriniai LT –

didelio elektronų

judrumo LT



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• Junction field effect transistors

• MOSFETs

• Parameters and equivalent circuits of FETs

• Frequency properties and parameters


Objectives:

Knowledge of field-effect transistors (FET):

• their structures and classification

• principles of operation

• static characteristics

• parameters and equivalent circuits

• frequency properties and parameters



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Structure and operation of JFETs

If reverse bias voltage is applied to the gates, the

space-charge layers extend into the channel.

Thus, application of a reverse voltage to the gates

governs the effective channel dimensions.

The depletion-layer thickness occurs in the n-type

channel region and the gate voltage effectively

controls the channel thickness if the p-type

gates are much more heavily doped than the

n-channel.

nn 2dbb −=( )

d

bGSn

qN

UU2ε2bb

+−=



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nn 2dbb −=( )

d

bGSn

qN

UU2ε2bb

+−=

If we increase the gate-source

voltage, the channel thickness

decreases. At some voltage the

thickness of the channel becomes

0. This voltage is called the pinch-

off voltage.

The gate voltage governs the

effective channel thickness, the

channel resistance and hence

controls the drain current. At the

pinch-off voltage the drain current

becomes 0. The pinch-off voltage

is negative for n-channel devices

and positive for p-channel JFETs.



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The pinch-off voltage

bdn

P

qU

8ε

NbU

2

−=

+

+−=

bP

bGSn

UU

UU1bbDrain current versus gate-source

voltage



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The drain current is

dependent on the drain

voltage. It increases with

increase of the drain

voltage.

The thickness of the

conducting channel at the

drain end decreases if the

drain voltage increases.

At some drain voltage the

space-charge regions from

the gates meet. The channel

becomes pinched-off. At

drain voltages beyond

pinch-off, the drain current

becomes saturated and

remains almost at some

value.




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…Increase of the output voltage drops on the

depletion layer and causes electric field. The

field causes the drift of charge carriers to drain

region.




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1. JFETs are voltage-controlled devices.

2. JFETs have high input impedance.

3. There are four regions of JFET operation: the ohmic, saturation,

breakdown and cut-off regions.

4. The input current of a JFET is small. The drain current is dependent on

the input and output voltages. Therefore a JFET can be characterized by

one set of I-U characteristics. In practice two sets are used: the common-

source transfer curves (perdavimo charakteristikos) and the common-

source output curves - drain characteristics (išėjimo charakteristikos).




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2

0GS

GSmaxDD 1

−=U

UII


In the pinch-off or saturation region the relationship between the output

current and input voltage is non-linear and is defined by Shockley’s

equation:



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Structures of JFETs

Structure of (a) n-channel JFET and (b) MESFET

If the drain current increases, the slope of the transfer characteristic also

increases and higher gain is possible.

However the gate-source voltage must be reverse to the gate junction. This

condition limits increasing of drain current and gain.

… pn junction can be changed by dielectric layer.



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IG FETs, MISFETs,

MOSFETs(LT su izoliuotąja užtūra

– MDP LT, MOP LT)

E-MOSFETs(LT su indukuotuoju

kanalu - praturtintosios

veikos LT)

DE-MOSFETs(LT su įterptuoju kanalu – nuskurdintosios-praturtintosios

veikos LT)

HEMTs(Heterostruktūriniai LT –

didelio elektronų

judrumo LT)

MOSFETs



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DE-MOSFETs

DE-MOSFET – depletion-

enhancement MOSFET

2

P

GSDSsD 1

−=U

UII



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DE-MOSFETs (with induced channel)



18

2GS0GSS )( UUKI −=

DE-MOSFETs (with induced channel)



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I-U characteristics of FETs



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FETs: parameters, models and frequency properties

;)( 20GSGSD UUAI −≅ DGS0GS

GS

Dm 2)(2 IAUUA

U

Ig =−==

∆∆

),(

0

DSGSD

G

UUfI

I

=

=

DSo

GSmD

1U

rUgI +=

DSo

GSmDSDS

DGS

GS

D d1

dddd Ur

UgUU

IU

U

IID +=

∂

∂+

∂

∂=

constatd

dDS

GS

Dm === U

U

ISg

constatd

dGS

D

DSo == U

I

Ur

-1CQm V 40q/k/ ≅≅ TIg )/(2/ GS0GSDQm UUIg −=>>



21

-1KQm V 40k/q/ ≅≅ TIg )/(2/ 0UIUISQm UUIg −=>>

mg ~kl

µ

The structure of a V-MOS

transistor

Isolated gate bipolar transistor (IGBT)

FETs: parameters, models and frequency properties



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The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-

current and low–saturation-voltage capability of bipolar transistors by combining an

isolated gate FET for the control input, and a bipolar power transistor as a switch, in a

single device.

The IGBT is a fairly recent invention. The first-generation devices of the 1980s and early

1990s were relatively slow in switching ...

Large IGBT modules typically consist of many devices in parallel and can have very high

current handling capabilities in the order of hundreds of amps with blocking voltages of

6,000 V. Toyota's second generation hybrid Prius has a 50 kW IGBT inverter controlling

two AC motor/generators connected to the DC battery pack.[

IGBT



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DG II =

τ2

1

)C(C2

gf

ππ GDGS

mT =

+=

maxch / vlτ =

ch

maxT

π2 l

vf =

o

mTmax

2 g

gff =

FETS: frequency properties

The slope of the transfer characteristic decreases with frequency:

x

x

x

S

SjK j

0

esin

)ω( −==2

ωτx =

The unit-gain condition is reached when the input current becomes equal to

the output drain current when the output is short-circuited.

GSmD UgI =GSGDGSG )(j UCCωI +=



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)(π2 GDGS

mT

1222DS

1211GS

12GD

CC

gf

CCC

CCC

CC

+=

−=

−=

=

constD

DSo

constGS

Dm

GS

DS

=

=

=

=

U

U

I

Ur

U

Ig

∆∆

∆∆

The Π type model of a FET

80 nm,… 400 GHz



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1. The charge carriers in an n-type channel of JFET are (holes, electrons).

2. As the charge carriers in a JFET move from the source region to the

drain region they cross (no, 1, 2) junctions.

3. An n-channel JFET is in its CS configuration. Its maximal drain current is

20 mA and the pinch-off voltage is minus 10 V. What is its drain current

when the gate-source voltage is zero volts? What value of output voltage

is required to saturate the JFET if the gate-source voltage is minus 2 V?

4. An n-channel JFET is in its CS configuration. Its pinch-off voltage is

minus 6 V and maximal drain current is 12 mA. Sketch the transfer

characteristic of the FET.

5. Negative values of input voltage are required to (enhance, deplete) the

channel of the n-channel DE-MOSFET while positive values of the

voltage are required to (enhance, deplete) the channel.



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6. Sketch the Π-type equivalent circuits of field effect and bipolar

transistors and comment on them.

7. A JFET is used in its CS configuration. Its maximal drain current is 8 mA,

the pinch-off voltage is minus 4 V. Find the slope of the transfer

characteristic at input voltage of minus 1,5 V.

8. A JFET is used in its CS configuration. Its maximal drain current is 9 mA,

the pinch-off voltage is minus 3 V. Find the slope of the transfer

characteristic at drain current of 4 mA. Sketch the Π-type model. Find

parameters of circuit elements. The input capacitance is 2 pF, the

transfer capacitance is 1 pF, and the output capacitance is 1.5 pF. Find

the gain-frequency product.

9. Explain why MOSFETs are typically shipped with their leads shortened

together.




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After years of research and experimentation involving literally

hundreds of scientist from around the world, the final breakthrough

in the development of the transistor was left to three men. Dr Walter

Brattain, Dr John Bardeen and Dr William Shockley all three

scientists working at Bell laboratories, are the men credited with this

significant achievement. In December 1947 they made the historic

discovery of the transistor effect and in so doing developed the very

first transistor device. In 1956 their achievement was acknowledged

when they were awarded the Nobel Prize for physics.



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___trailing the Transistor History.mht

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