1

Field-Effect Transistors The field-effect transistor (JFET) is the simplest type of field effect

transistors.

• Structure

• Current-voltage Characteristics

• MOSFET as an Amplifier

• CMOS

• The CMOS Digital Logic Inverter

2

n-channel Enhancement mode MOSFET Two n-regions called source terminal and drain terminal. The current in

a MOSFET is the result of flow of charge in the inversion layer called the

channel region.

Source Drain Channel

For

The channel width is uniform

4

iD-vDS Characteristics Cutoff; Triode (switch); and Saturation (amplifier) regions

Channel) (Induced tGS Vv

2mA/V 5.0'

V 1

L

Wk

V

n

t

5

To operate the MOSFET in the triode region

1

2

'

'

2

1'

tGSnD

DSDS

DStGSnD

DSDStGSnD

tGSDS

VvL

Wk

i

vr

vVvL

Wki

vvVvL

Wki

Vvv

6

To operate the MOSFET in the saturation region

DSnD

DSnD

tGSDS

tGSDS

tGD

tGS

vL

Wki

vL

Wki

Vvv

Vvv

Vv

Vv

2

2

'2

1

'2

1

mA/V2) 0.5nW/Lk’ V, 1 (

Saturationin r transistoNMOS type-tenhancemenan for

sticsCharacteri -

t

GSD

V

vi

7

iD-vGS characteristics for an enhancement-type NMOS

transistor in Saturation (Vt= 1 V, k’nW/L=0.5 mA/V2)

8

Increasing vDS beyond vDsat causes the channel pinch-off to

move away from the drain.

9

Effect of vDS on iD in the saturation region

10

Large-signal equivalent circuit model of the n-channel

MOSFET in saturation.

11

Exercise 5.3 • An Enhancement-type NMOS transistor with Vt = 2 V has its source

terminal grounded and a 3-V DC source connected to the gate. In what

region of operation does the device operate for:

• VD = +0.5 V (vDS vGS-Vt: Triode)

• VD = 1 V (vDS vGS-Vt: Saturation)

• VD = 5 V (vDS vGS-Vt: Saturation)

12

The Current-Voltage Characteristics of the Depletion n-Channel MOSFET

for Which Vt = -4V and k’nW/L = 2 mA/V2

13

The MOSFET as an Amplifier

DC Bias

tGSD

DDDDD

tGSnD

VVV

IRVV

VVL

WkI

Saturation Ensure To

'2

1 2

14

The Signal Current in the Drain Terminal

tGSngs

dm

gstGSnd

dDD

tgsGSnD

gsGSGS

VVL

Wk

v

ig

vVVL

Wki

iIi

VvVL

Wki

vVv

'

'

'2

1 2

15

16

Small Signal Equivalent Circuit

17

The T Model

18

19

Basic Configurations of Single-Stage IC MOS

Amplifiers

• Common Source (CS)

• Common Gate (CG)

• Common Drain (CD)

20

Amplifier Source-Common CMOS

21

2

2

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REF

A

o

rrgv

vA

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21

Amplifier Gate-Common CMOS The

1

2

11

2111

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mbmi

i

oombmv

ooo

mbmi

ov

r

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22

AmplifierFollower -Sourceor Drain -Common The

2111

11

1

2111

1

1

1

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1

oombmo

mbm

m

oombm

mv

Sm

Sm

i

ov

rrggR

gg

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23

Loadt Enhancemenwith Amplifier NMOS

24

LoadDepletion with Amplifier NMOS The

25

CMOS Inverter

26

High is hen Inverter W CMOS theofOperation Iv

tnDD

n

nDS VVL

Wkr '/1

27

Low is hen Inverter W CMOS theofOperation Iv

tpDD

p

pDSP VVL

Wkr '/1

28

The Voltage Transfer Characteristics of the CMOS Inverter

and Noise Figures

OLILML

IHOHMH

tDDIL

tDDIH

VVN

VVN

VVV

VVV

238

1

258

1

nn

pp

thtpDD

th

LWk

LWkr

r

VVVrV

/'

/'

1

29

Operation Dynamic

Delayn Propagatio2

High toLow/'

6.1

Low High to/'

6.1

PLHPHLp

DDpnPLH

DDnnPHL

ttt

VLWk

Ct

VLWk

Ct