cmos analog design lect 3

EE 290C

CMOS Analog Design Using All-region MOSFET Modeling

Lecture 3: Charge control compact MOSFET

model

CMOS Analog Design Using All-Region MOSFET Modeling 2

The Unified Charge Control Model (UCCM) 1

1 1I C

ox b i

dQ dVC C C

′ + =

′ ′ ′+

Approximations:

1) depletion capacitance per unit area is constant along the

channel and is calculated neglecting inversion charge

2) Charge sheet model /i I tC Q φ′ ′= −

The Unified Charge Control Model

(UCCM) 2

CMOS Analog Design Using All-Region MOSFET Modeling 3

1 1I C

ox b i

dQ dVC C C

′ + =

′ ′ ′+

/i I tC Q φ′ ′= −

1 tI C

ox I

dQ dVnC Q

φ ′ − =

′ ′

( )1 bGB

ox

Cn n V

C

′= + =

′where

Integrating from an arbitrary channel potential VC to a

reference potential VP yields the unified charge control

model (UCCM)

lnIP I IP C t

ox t IP

Q Q QV V

nC Qφ

φ

′ ′ ′−− = +

′ ′ is the value of

for V=VP. IPQ′

IQ′

The “Regional” Strong and Weak

Inversion Approximations

CMOS Analog Design Using All-Region MOSFET Modeling 4

lnIP I IP C t

ox t IP

Q Q QV V

nC Qφ

φ

′ ′ ′−− = +

′ ′

( )I ox P CQ nC V V′ ′− ≅ −

P C tV V φ− � P C tV V φ− −�

ln 1IP C t

IP

QV V

Qφ ′

− ≅ − ′

or, equivalently

P C t

t

V V

I IPQ Q e

φ

φ

− +

′ ′=

strong inversion weak inversion

Example

CMOS Analog Design Using All-Region MOSFET Modeling

5

(a) Calculate the value of the inversion charge density,

normalized to , for which the value of the voltage

VP-VS calculated using the SI approximation differs from

that calculated using UCCM by 10 %; (b) Same using

the WI approximation ; (c) comment on ‘moderate’

inversion (MI)

(b) Answer: a)

SI approximation error of less than 10 % for q’I > 20

b) WI approximation

WI approximation error of less than 10 % for q’I < 0.22.

(c) MI region : SI and WI approximations give errors greater

than 10 % for the control voltage VP-VS. The inversion

charge density variation from the lower to the upper limit of

the MI region is approximately two orders of magnitude

(20/0.22).

ox tnC φ′−

( ) /I P C t

q V V φ′ ≅ −

P C t

t

V V

Iq e

φ

φ

− +

′ =

The Pinch-off Voltage Charge Density

CMOS Analog Design Using All-Region MOSFET Modeling 6

The channel charge density corresponding to the

effective channel capacitance times the thermal

voltage, or thermal charge, defines pinch-off

( )IP ox b t ox t

Q C C nCφ φ′ ′ ′ ′= − + = −

The name pinch-off is retained herein for historical

reasons and means the channel potential

corresponding to a small (but well-defined)

amount of carriers in the channel.

The Pinch-off Voltage VP

CMOS Analog Design Using All-Region MOSFET Modeling 7

The channel-to-substrate voltage (VC) for which the

channel charge density equals is called the pinch-off

voltage VP.

UCCM is asymptotically

correct in weak inversion if

in weak

inversion( ) ( )2 / 2 /

( 1)sa F C t sa F C tV V

I b t ox tQ C e C n eφ φ φ φ φ φ

φ φ− − − −′ ′ ′− = = −

2 1 ln1

P sa F t

nV

nφ φ φ

= − − + −

2P sa FV φ φ≅ −

Threshold Voltage

Equilibrium threshold

voltage VT0, for VC=0,

gate voltage for which

Q’I = Q’IP = -nC’oxφt

or gate voltage for which

VP=0 2

P sa FV φ φ≅ −

G FB sa ox sa tV V Cφ γ φ φ′− = + −Recalling that

0 2 2 T FB F FV V φ γ φ≅ + +it follows that

8CMOS Analog Design Using All-Region MOSFET Modeling

Example

CMOS Analog Design Using All-Region MOSFET Modeling

9

Estimate VT0 for an n-channel transistor with n+ polysilicon

gate, NA=1017 atoms/cm3 and tox=5 nm.

Answer:

(a) The flat-band voltage (Lec 2) is -0.98 V; φF=0.419;

C’ox= 690 nF/cm2.

For this low value of the threshold voltage, the off-current (for

VGS=0) is too high for digital circuits. Solution to control the

magnitude of the threshold voltage without an exaggerated

increase in the slope factor -> a non-uniform high-low channel

doping.

2 / 0.264 Vs A oxq N Cγ ε ′= =

0 0.98 0.838 0.264 0.838= 0.1VTV ≅ − + +

CMOS Analog Design Using All-Region MOSFET Modeling n

VVV 0TGBP

−−−−≅≅≅≅

Pinch-off Voltage vs. Gate Voltage

Useful approximation:

-1.00E+00

0.00E+00

1.00E+00

2.00E+00

3.00E+00

4.00E+00

0.00E+00 1.00E+00 2.00E+00 3.00E+00 4.00E+00 5.00E+00 6.00E+00

pinch-off voltage

0

0.5

1

1.5

2

slope factor

4.0

3.0

2.0

1.0

0

-1.0

0 1.0 2.0 3.0 4.0 5.0

2.0

1.5

1.0

0.5

0

VG (V)

VP

VT0 (equilibrium threshold voltage)

1sa oxP

G G b ox

d CdV

dV dV C C n

φ ′≅ = =

′ ′+

10

MOS Transistor

0 0 0

i ix xW

D n nI J dxdz W J dx= − = −∫ ∫ ∫

11CMOS Analog Design Using All-Region MOSFET Modeling

‘Exact’ I-V Model of the MOSFET 1

CMOS Analog Design Using All-Region MOSFET Modeling 12

n nn

d dnJ qn qD

dy dy

φµ

= − +

( )

0 0

C

C

q V

u ukTn n e n e

φ −−= =

C

t

dVdn n d

dy dy dy

φ

φ

= −

Using the Einstein

relationship

n n tD µ φ= C Cn n n n

dV dVd dJ qn qn qn

dy dy dy dy

φ φµ µ µ

= − + − = −

drift +diffusion current

S C DV V V≤ ≤

‘Exact’ I-V Model of the MOSFET 2

CMOS Analog Design Using All-Region MOSFET Modeling 13

0 0 0

i ix xW

D n nI J dxdz W J dx= − = −∫ ∫ ∫

0

xiC C

D n In

dV dVI qW n dx QW

dy dyµ µ ′= = −∫

Cn n

dVJ qn

dyµ= −

0

xi

IQ q ndx′ = − ∫

D

S

Vn

D I CV

WI Q dV

L

µ′= − ∫

L is the channel length

Since the current is constant along the channel

CMOS Analog Design Using All-Region MOSFET Modeling 14

IC s t

I

dQdV d

Qφ φ

′= −

′

( )I i C sdQ C dV dφ′ ′= −

CdVsdφ

GVI

i

t

QC

φ

′′ = −

IdQ′+ _

Charge-Sheet Formula for the Current

D drift diff

s In I n t

I I I

d dQWQ W

dy dy

φµ µ φ

= + =

′′− +

Cn n

dVJ qn

dyµ= −

Charge Control Compact Model 1

sdφ

BdQ′

+_

GV

bC′ +

_

IdQ′

iC′

oxC′

CdV

( )I ox b s ox sdQ C C d nC dφ φ′ ′ ′ ′= + =

s ID n I n t

d dQI WQ W

dy dy

φµ µ φ

′′= − +

( )n ID I t ox

ox

W dQI Q nC

nC dy

µφ

′′ ′= − −

′

Integrating along the channel yields

( )2 2

2

n IS IDD t IS ID

ox

W Q QI Q Q

L nC

µφ

′ ′−′ ′= − −

′ 15CMOS Analog Design Using All-Region MOSFET Modeling

MOSFET Modeling for Circuit Analysis and Design

16

Charge Control Compact Model - 2

( )2 2

2 2

n IS ID IS ID IS IDD t IS ID ox tn

ox ox

W Q Q Q Q Q QI Q Q W nC

L nC nC L

µφ φµ

′ ′ ′ ′ ′ ′− + − ′ ′ ′= − − = − ′ ′

drift + diffusion

0

2

IS ID s sLD ox tn

Q QI W nC

L

φ φφµ

′ ′+ − ′= −

average

charge average field

“virtual” charge

Charge Control Compact Model 3

To emphasize the symmetry of the

rectangular geometry MOSFET

D F RI I I= −

2( )

( ) ( )2

IS D

F R t IS Dnox

QWI Q

L nCφµ

′′= −

′

(compare with Ebers-Moll model of the BJT)

17CMOS Analog Design Using All-Region MOSFET Modeling

MOSFET Modeling for Circuit Analysis and Design 18

Drain Current vs. Gate-to-Bulk

Voltage

MOSFET Modeling for Circuit Analysis and Design 19

Comparing UCCM and the Surface Potential

Model with Exact Numerical Solution of

Poisson Equation

Modeling the Bulk Charge from

Accumulation to Inversion

CMOS Analog Design Using All-Region MOSFET Modeling

20

/sgn( ) ( 1)s t

B s ox s tQ C eφ φφ γ φ φ −′ ′= − + −

( )( ) ( )

/1

1 12sgn 1

sa t

sa t

b

oxsa sa t

eCn

C e

φ φ

φ φ

γ

φ φ φ

−

−

−′= + = +

′ + −

( ) ( )[ ]122−+=−− − tsaeVV tsasaFBG

φφφφγφ

Modeling from Accumulation to Inversion: Surface Potential and Pinch-off Voltage (VP)

CMOS Analog Design Using All-Region MOSFET Modeling 21