Unit1 ch-01.ppt

Basic MOS Device Physics
Chapter 1

MOS Device Structure
Fig: Simple MOS Device
1. MOSEFT as a SWITCH
2. MOSFET Structure
Fig1: Structure of a MOS Device

NMOS and PMOS with Well
Fig2: (a): Simple PMOS device and (b): PMOS inside an n-well

MOS Symbols
Fig: MOS Symbols

MOS I/V Characteristics
1. Threshold Voltage

Derivation of I/V Characteristics
)
1
(






  v
Q
I d

I/V Characteristics

I/V Characteristics
)
2
(
)
( 






 TH
GS
ox
d V
V
WC
Q
)
1
(






  v
Q
I d
)
3
(
)
)
(
(
)
( 






 TH
GS
ox
d V
x
V
V
WC
x
Q

I/V Characteristics (cont.)
)
4
(
]
)
(
[ 






 v
V
x
V
V
WC
I TH
GS
ox
D
)
6
(
]
)
(
[
0
0






 
 

VDS
V
TH
GS
n
ox
L
x
D dV
V
x
V
V
WC
dx
I 
Given v  E and E(x)  
dV (x)
dx
)
5
(
)
(
]
)
(
[ 







dx
x
dV
V
x
V
V
WC
I n
TH
GS
ox
D 
)
7
(
]
2
1
)
[( 2






 DS
DS
TH
GS
L
W
ox
n
D V
V
V
V
C
I 
From eq(1) the current is given by
Using Boundary conditions V(0)=0 and V(L)=Vds

I/V Characteristics (cont.)
ID  nCox
W
L
[(VGS  VTH)VDS 
1
2
VDS
2
]

Operation in Triode Region
)
8
(
2
)
[(
2
1




 TH
GS
L
W
ox
n
D V
V
C
I 
)
9
(
)
( 



 DS
TH
GS
L
W
ox
n
D V
V
V
C
I 
)
10
(
)
(
1




TH
GS
L
W
ox
n
ON
V
V
C
R

If Vds=Vgs-Vth
)
(
2 TH
GS
DS V
V
V 

In eqn 7

Operation in Active (Saturation) Region

11
)
)
(
(
)
( 






 TH
GS
ox
d V
x
V
V
WC
x
Q
Using Boundary conditions for saturated device
)
12
(
]
)
(
[
0
0






 
 

VDS
V
TH
GS
n
ox
L
x
D dV
V
x
V
V
WC
dx
I 
)
13
(
]
)
(
[
0
0






 





VTH
VGS
V
TH
GS
n
ox
L
x
D dV
V
x
V
V
WC
dx
I 
Limits of left hand side from eqn (12) x=0 to x=L’ and right
hand side frrom V(x)=0 to V(x)=Vgs- Vth

Active Region (cont.)
)
14
(
)
( 



 
DS
TH
GS
L
W
ox
n
D V
V
V
C
I 

Transconductance, gm
gm 
ID
VGS VDS cons tan t
 nCox
W
L
(VGS  VTH)
gm  2nCox
W
L
ID

2ID
VGS  VTH

Triode and Active Region Transition
Active Active

Second Order effects
1.Body Effect

Threshold Voltage and Body Effect (cont.)
VTH  VTH0  2F  VSB  2F
 ,  
2qsiNsub
Cox
No Body Effect With Body Effect
,
2
,
ox
dep
F
MS
TH
C
Q
V
Where 



 MS  gat e  sili con
 ,
ln
i
sub
F
n
N
q
kT 






 Qdep  4qsi F Nsub

2. Channel Length Modulation
L'  L L 1/ L' 
1
L
(1  L/ L)
1/ L' 
1
L
(1  VDS), VDS  L/ L
ID 
nCox
2
W
L
(VGS  VTH)2
(1  VDS)
L L’

Channel Length Modulation (cont.)
gm  nCox
W
L
(VGS  VTH)(1  VDS)
gm 
2nCox W
LID
(1  VDS)
gm 
2ID
VGS  VTH
, (unchanged)

3. Subthreshold Conduction
ID  I0 exp
VGS
 kT
q







MOS Layout

MOS Device Capacitances

Layout for Low Capacitance

G-S and G-D Capacitance
Different regions of MOS device capacitance

2. MOS Small Signal Models

Bulk Transconductance, gmb
gmb 
ID
VBS

nCox
2
W
L
(VGS  VTH)
VTH
VBS




gmb  gm

2 2F VSB
 gm
Also,
VTH
VBS

VTH
VSB
 

2
(2F  VSB)1/ 2
ro 
VDS
ID

1
ID /VDS

1
nCox
2
W
L
(VGS  VTH)2


1
ID

MOS Small Signal Model with Capacitance

Unit1 ch-01.ppt

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Unit1 ch-01.ppt