spring 2007ee130 lecture 38, slide 1 lecture #38 outline the mosfet: bulk-charge theory body effect...

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


Problem with the “Square Law Theory”

• Ignores variation in depletion width with distance y

CSTGoxeinv VVVVCQ


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


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 /


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


MOSFET VT Measurement

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

IDS

VGS


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


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)


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


Small Signal Model

• Conductance parameters:

)(

0

TGSoxeeff

constVG

Dm

Dsat

constVD

Dd

VVmL

CW

V

Ig

IV

Ig

D

G

gmddd vgvgi


Inclusion of Additional Parasitics


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

spring 2007ee130 lecture 38, slide 1 lecture #38 outline the mosfet: bulk-charge theory body effect...

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