ECE340

ELECTRONICS I

MOSFET TRANSISTORS AND AMPLIFIERS

MOSFET

• METAL-OXIDE-SEMICONDUCTOR FIELD EFFECT TRANSISTOR

• VOLTAGE - CONTROLLED DEVICE

• LOW POWER DISSIPATION

MOSFET

SOURCE DRAIN

OXIDE

METAL

OXIDE OXIDE

CHANNEL

L

NMOSFET ENHANCEMENT MODE DEVICE

N TYPE SOURCE N TYPE DRAIN

OXIDE

METAL

OXIDE OXIDE

P TYPE SUBSTRATE

+VG+VD

-VS

-VB

DEPLETION LAYER DEPLETION LAYER

MOSFET “ON” CONDITION

n+ n+

OXIDE

METAL

OXIDE OXIDE

VG > VTN +VD

p

ID

electrons

MOSFET PARAMETERS

• iD – DRAIN CURRENT

• VTP,VTN – THRESHOLD VOLTAGE (VTH)

• vDS – DRAIN TO SOURCE VOLTAGE

• vGS – GATE TO SOURCE VOLTAGE

• vB – BULK VOLTAGE

THRESHOLD VOLTAGE

• VOLTAGE REQUIRED TO CREATE AN INVERSION LAYER OF CHARGE UNDER THE GATE OXIDE

• POSITIVE FOR n-CHANNEL DEVICES

• NEGATIVE FOR p-CHANNEL DEVICES

BULK VOLTAGE

• LOWEST VOLTAGE AVAILABLE FOR NMOS (N-CHANNEL) DEVICES

• HIGHEST VOLTAGE AVAILABLE FOR PMOS (P-CHANNEL) DEVICES

• REVERSE-BIASES PN JUNCTIONS

MOSFET CAPACITANCE

• POSITIVE OR NEGATIVE VOLTAGE AT GATE TERMINAL INDUCES CHARGE ON GATE METAL

• CHARGE OF OPPOSITE TYPE ACCUMULATES IN CHANNEL

• FORMS MOSFET CAPACITOR

OXIDE CAPACITANCE

cmF

tC

o

oox

ox

oxox

/1085.8

9.3

14

PARAMETER DEFINITIONS

n,p - ELECTRON OR HOLE MOBILITY

ox – PERMITTIVITY OF OXIDE

• tox – OXIDE THICKNESS

• (W/L) – ASPECT RATIO

MOSFET OPERATION

• SOURCE TERMINAL IS GROUNDED

• GATE AND DRAIN VOLTAGES REFERENCED TO SOURCE VOLTAGE

• VOLTAGE IS APPLIED TO GATE TERMINAL TO INDUCE CHARGE IN THE CHANNEL

CHARGE FLOW

• CHARGE IS PULLED INTO CHANNEL FROM DRAIN AND SOURCE REGIONS

• CHARGE FLOWS FROM SOURCE TO DRAIN AS DRAIN VOLTAGE IS INCREASED

DEVELOPMENT OF MOSFET EQUATIONS

dt

dxchargechannelofvelocity

dx

xdvxEv

VxvvWdxCdq

DS

THGSox

DEVELOPMENT OF MOSFET EQUATIONS

dx

xdvVxvvWCμiii

dt

dx

dx

dqi

dt

dqi

dt

dxchargechannelofvelocity

THGSoxnDD

DEVELOPMENT OF MOSFET EQUATIONS

DSv

THGSoxn

L

D

THGSoxnD

THGSoxnDD

xdvVxvvWCdxi

xdvVxvvWCdxi

dx

xdvVxvvWCiii

00

DEVELOPMENT OF MOSFET EQUATIONS

2'

'

2

2

1

2

1

DSDSTHGSnD

oxnn

DSDSTHGSoxnD

vvVvL

Wki

Ck

vvVvL

WCi

N-CHANNEL MOSFET EQUATIONS

2'

2'

2

1

2

1

0

THGSnDTHGSDS

DSDSTHGSnDTHGSDS

DTHGS

VvL

WkiVvv

vvVvL

WkiVvv

iVv

MOSFET CHARACTERISTICS

vDS

0V 2V 4V 6V 8V 10V 12V

ID

0mA

0.5mA

1.0mA

1.5mA

vGS3

vGS2

vGS1

TRANSCONDUCTANCE PARAMETER COMPONENTS

• MOBILITY

• ELECTRIC PERMITTIVITY

• OXIDE THICKNESS

• ASPECT RATIO

TRANSCONDUCTANCE PARAMETER PHYSICS

ox

oxox

oxox

ox

tC

Ckt

k

L

WkK

''

'

n-CHANNEL MOSFET OPERATION IN CUTOFF REGION

0

D

THGS

i

Vv

n-CHANNEL MOSFET OPERATION IN LINEAR REGION

2

2

1DSDSTHGSnD

THGSDS

vvVvKi

Vvv

n-CHANNEL MOSFET OPERATION IN SATURATION REGION

22

1THGSnD

THGSDS

VvKi

Vvv

p-CHANNEL MOSFET OPERATION IN CUTOFF REGION

0

D

THSG

i

Vv

p-CHANNEL MOSFET OPERATION IN LINEAR REGION

2

2

1SDSDTHSGpD

THSGSD

vvVvKi

Vvv

p-CHANNEL MOSFET OPERATION IN SATURATION REGION

22

1THSGpD

THSGSD

VvKi

Vvv

NMOS INCREMENTAL RESISTANCE IN THE LINEAR REGION

1

1

2

1

2

1

THGSnDS

smallv

VvDS

DDS

DSTHGSnDDS

DSDSTHGSnD

VVKrv

ir

vVvKiv

vvVvKi

DS

GSGS

PMOS INCREMENTAL RESISTANCE IN THE LINEAR REGION

1

1

2

1

2

1

THSDnDS

smallv

VvSD

DSD

SDTHSGpDDS

SDSDTHSGpD

VVKrv

ir

vVvKiv

vvVvKi

SD

SGSG

MODULATED CHANNEL IN SATURATION REGION

n+ n+

OXIDE

METAL

OXIDE OXIDE

VG > VTN +VD

p

ID

VD>>VG

TAPERED CHANNEL

NMOS INCREMENTAL RESISTANCE IN SATURATION REGION

1

12

1

2

2

12

1

DO

THGSn

O

VvDS

DO

DSTHGSnD

Ir

VVK

rv

ir

vVvKi

GSGS

PMOS INCREMENTAL RESISTANCE IN SATURATION REGION

1

1

2

1

2

2

12

1

DO

THSGp

O

VvSD

DO

SDTHSGpD

Ir

VVK

rv

ir

vVvKi

SGSG

DEPENDENCE ON DRAIN VOLTAGE

2THGS'nD

DO

VVL

Wk

2

1I

parametermodulationlengthchannelλ

λI

1r

PSPICE MOSFET SYMBOLS

p-channel enhancement n-channel enhancement

NMOS LARGE SIGNAL MODEL

VGS

VDS

+

-

S

+

G

S

rO

-

DG

2THGS'n VV

L

Wk

2

1

DEVELOPMENT OF MOSFET SMALL-SIGNAL MODEL

dDDgsGSGS iIivVv

TOTAL CURRENT AND VOLTAGE

gsTHGSTHGSnDgs

gsgsTHGSTHGSnD

gsTHGSnDTNgsGSnD

vVVVVL

Wkiv

vvVVVVL

Wki

vVVL

WkiVvV

L

Wki

22

11

22

1

2

1

2

1

2'2

22'

2'2'

COMPONENTS OF TOTAL CURRENT

gsTHGSnd

gsTHGSnTHGSnD

dDD

vVVL

Wki

vVVL

WkVV

L

Wki

iIi

'

'2'

2

1

MOSFET TRANSCONDUCTANCE

THGSnm

gs

dm

gsTHGSnd

VVL

Wkg

v

ig

vVVL

Wki

'

'

ALTERNATIVE TRANSCONDUCTANCE EQUATION

DnmTHGSnm

n

DTHGSTHGSnD

IL

WkgVV

L

Wkg

LW

k

IVVVV

L

WkI

''

'

2'

2

2

2

1

SMALL-SIGNAL MODEL

g d

s s

rO

VCC

vds

+

-

+

vgs

-

gmvgs

id