EE130 Lecture 38, Slide 1Spring 2007

Lecture #38

OUTLINE

The MOSFET:• Bulk-charge theory• Body effect parameter• Channel length modulation parameter• PMOSFET I-V • Small-signal model

Reading: Finish Chapter 17, 18.3.4

EE130 Lecture 38, Slide 2Spring 2007

Problem with the “Square Law Theory”

• Ignores variation in depletion width with distance y

CSTGoxeinv VVVVCQ

EE130 Lecture 38, Slide 3Spring 2007

Modified (Bulk-Charge) Model

T

oxe

oxe

dm

W

T

C

Cm

311 where

23 since OSiSi

DSDSTGeffoxeDlin VVm

VVCL

WI )

2(

2)(2 TGeffoxeDsat VVC

mL

WI

• saturation region:m

VVVV TG

DsatD

• linear region:m

VVVV TG

DsatD

EE130 Lecture 38, Slide 4Spring 2007

The expression that was previously derived for VT is the

gate voltage referenced to the body voltage that is required reach the threshold condition:

MOSFET Threshold Voltage, VT

ox

SBFSiAFSBFBT C

VqNVVV

)2(22

Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case,

and the equations for IDS areox

SBFSiAFFBT C

VqNVV

)2(22

DSDSTGSeffoxeDlin VVm

VVCL

WI )

2( 2)(

2 TGSeffoxeDsat VVCmL

WI

mVVVV TGSDSsatDS / mVVVV TGSDSsatDS /

EE130 Lecture 38, Slide 5Spring 2007

Note that VT is a function of VSB:

The Body Effect

where is the body effect parameter

When the source-body pn junction is reverse-biased, |VT| is increased. Usually, we want to minimize so that IDsat will be the same for all transistors in a circuit

FSBFTFSBFox

SiAT

ox

SBFSiA

ox

FSiA

ox

FSiAFFB

ox

SBFSiAFFBT

VVVC

qNV

C

VqN

C

qN

C

qNV

C

VqNVV

22222

)2(2)2(2)2(22

)2(22

00

EE130 Lecture 38, Slide 6Spring 2007

MOSFET VT Measurement

• VT can be determined by plotting IDS vs. VGS, using a low value of VDS

IDS

VGS

EE130 Lecture 38, Slide 7Spring 2007

Channel Length Modulation Parameter, • Recall that as VDS is increased above VDsat, the width L of

the depletion region between the pinch-off point and the drain increases, i.e. the inversion layer length decreases.

L

L

LLLIDsat 1

11

DSsatDS VVL

DSsatDS VVL

L

DSsatDSTGSeffoxeDsat VVVVCmL

WI 1)(

22

EE130 Lecture 38, Slide 8Spring 2007

P-Channel MOSFET• The PMOSFET turns on when VGS < VTp

– Holes flow from SOURCE to DRAIN DRAIN is biased at a lower potential than the SOURCE

• In CMOS technology, the threshold voltages are usually symmetric: VTp = -VTn

P+ P+

N

GATEVS VD

VG

IDS

VB

• VDS < 0

• IDS < 0

• |IDS| increases with

• |VGS - VTp|

• |VDS| (linear region)

EE130 Lecture 38, Slide 9Spring 2007

DSDSTpGSeffpoxeDS VVm

VVCL

WI )

2(,

PMOSFET I-V

• Linear region:

• Saturation region:

2, )(

2 TpGSeffpoxeDsatDS VVCmL

WII

m

VVV

TpGS

DS

0

m

VVV

TpGS

DS

m = 1 + (3Toxe/WT) is the bulk-charge factor

EE130 Lecture 38, Slide 10Spring 2007

Small Signal Model

• Conductance parameters:

)(

0

TGSoxeeff

constVG

Dm

Dsat

constVD

Dd

VVmL

CW

V

Ig

IV

Ig

D

G

gmddd vgvgi

EE130 Lecture 38, Slide 11Spring 2007

Inclusion of Additional Parasitics

EE130 Lecture 38, Slide 12Spring 2007

Cutoff Frequency

• fmax is the frequency where the MOSFET is no longer amplifying the input signal– Obtained by considering the small-signal model

with the output terminals short-circuited, and finding the frequency where |iout / iin| = 1

Increased MOSFET operating frequencies are achieved by decreasing the channel length

LVV

mL

W

C

gf TGS

eff

oxe

m 1)(

22max