Lecture 10
MOSFET (III)
MOSFET Equivalent Circuit Models
Outline
• Low-frequency small-signal equivalent circuit
model
• High-frequency small-signal equivalent circuit
model
Reading Assignment:
Howe and Sodini; Chapter 4, Sections 4.5-4.6
Announcements:
1. Quiz#1: March 14, 7:30-9:30PM, Walker Memorial;
covers Lectures #1-9; open book; must have
calculator
• No Recitation on Wednesday, March 14: instructors or
TA’s available in their offices during recitation times
6.012 Spring 2007
Lecture 10
1
Large Signal Model for NMOS Transistor
Regimes of operation:
VDSsat=VGS-VT
ID
linear
saturation
ID
VDS
VGS
VGS
VBS
VGS=VT
0
0
•
Cut-off
VDS
cutoff
ID = 0
•
Linear / Triode:
ID =
•
W
V
µn Cox ⎡ VGS − DS − VT ⎤ • VDS
⎣
⎦
L
2
Saturation
I D = I Dsat =
W
2
µn Cox [VGS − VT ] • [1 + λVDS ]
2L
Effect of back bias
VT (VBS ) = VTo + γ
6.012 Spring 2007
[ −2φ
p
− VBS − −2φp
Lecture 10
]
2
Small-signal device modeling
In many applications, we are only interested in the response
of the device to a small-signal applied on top of a bias.
ID+id
vgs +
-
VGS
+
vds
+
v
- bs
VBS
VDS
Key Points:
•
Small-signal is small
– ⇒ response of non-linear components becomes
linear
•
•
Since response is linear, lots of linear circuit
techniques such as superposition can be used to
determine the circuit response.
Notation: iD = ID + id ---Total = DC + Small Signal
6.012 Spring 2007
Lecture 10
3
Mathematically:
i D (VGS , VDS , VBS ; v gs , vds , vbs ) ≈
I D (VGS , VDS , VBS ) + id (v gs , v ds , vbs )
With id linear on small-signal drives:
id = gm vgs + go vds + gmb vbs
Define:
gm ≡ transconductance [S]
go ≡ output or drain conductance [S]
gmb ≡ backgate transconductance [S]
Approach to computing gm, go, and gmb.
gm ≈
∂i D
∂vGS Q
go ≈
∂i D
∂vDS
g mb ≈
Q
∂i D
∂vBS
Q
Q ≡ [vGS = VGS, vDS = VDS, vBS = VBS]
6.012 Spring 2007
Lecture 10
4
Transconductance
In saturation regime:
W
2
iD =
µn Cox [vGS − VT ] • [1 + λVDS ]
2L
Then (neglecting channel length modulation) the
transconductance is:
gm =
∂i D
∂vGS
≈
Q
W
µn Cox (VGS − VT )
L
Rewrite in terms of ID:
gm = 2
gm
W
µ nCox ID
L
saturation
0
0
6.012 Spring 2007
ID
Lecture 10
5
Transconductance (contd.)
Equivalent circuit model representation of gm:
id
G
vgs
S
D
+
gmvgs
-
B
6.012 Spring 2007
Lecture 10
6
Output conductance
In saturation regime:
W
2
iD =
µn Cox [vGS − VT ] • [1 + λVDS ]
2L
Then:
go =
W
∂iD
2
=
µnCox (VGS − VT ) • λ ≈ λID
∂v DS Q 2L
Output resistance is the inverse of output conductance:
ro =
1
1
=
g o λID
Remember also:
1
λ∝
L
Hence:
ro ∝ L
6.012 Spring 2007
Lecture 10
7
Output conductance (contd.)
Equivalent circuit model representation of go:
id
G
vgs
S
D
+
ro
-
B
6.012 Spring 2007
Lecture 10
8
Backgate transconductance
In saturation regime (neglect channel length modulation):
iD ≈
W
2
µn Cox [vGS − VT ]
2L
Then:
g mb
⎛ ∂V ⎞
∂i D
W
=
= − µn Cox (VGS − VT ) • ⎜ T ⎟
L
∂v BS Q
⎝ ∂v BS Q ⎠
Since:
VT (vBS ) = VTo + γ
[ −2φ
p
− v BS − −2φp
]
Then :
∂VT
∂vBS
=
Q
−γ
2 −2φp − VBS
Hence:
g mb =
6.012 Spring 2007
γ gm
2 −2φ p − VBS
Lecture 10
9
Backgate transconductance (contd.)
Equivalent circuit representation of gmb:
id
G
vgs
S
D
+
gmbvbs
-
vbs
B
6.012 Spring 2007
+
Lecture 10
10
Complete MOSFET small-signal
equivalent circuit model for low frequency:
id
G
D
+
vgs
S
gmbvbs
gmvgs
ro
-
vbs
B
+
;;;
;;
;;;;
;
;;;;
;;;;
;;;;
;
;
;
;;;
;;;;
+ V
DS
−
+ V
GS
−
metal
interconnect to gate
n+ polysilicon gate
n+ source
VBS
+
−
0
x
y
QN (y)
n+ drain
Xd (y)
p-type
;;;
;;;
;;;
;;;
;;;
;;;
;;;
;;;
;
metal interconnect to bulk
6.012 Spring 2007
Lecture 10
11
2. High-frequency small-signal
equivalent circuit model
;;
Need to add capacitances. In saturation:
fringe electric
field lines
gate
drain
;;;
;
;;
;;;
source
n+
n+
Csb
qN (vGS)
overlap LD
overlap LD
Cdb
depletion
region
Cgs ≡ channel charge + overlap capacitance, Cov
Cgd ≡ overlap capacitance, Cov
Csb ≡ source junction depletion capacitance (+sidewall)
Cdb ≡ drain junction depletion capacitance (+sidewall)
ONLY Channel Charge Capacitance is intrinsic to device
operation. All others are parasitic.
6.012 Spring 2007
Lecture 10
12
Inversion layer charge in saturation
L
vGS −VT
0
0
qN (vGS ) = W ∫ QN (y)dy = W
∫
dy
QN (vC )•
• dvC
dvC
Note that qN is total inversion charge in the channel &
vC(y) is the channel voltage. But:
dvC
iD
=−
dy
W µnQN (vC )
Then:
W 2 µn
qN (vGS ) = −
•
iD
VGS −VT
∫
[QN (vC ) ] • dvC
2
0
Remember:
QN (vC ) = −Cox [vGS − vC (y) − VT ]
Then:
W 2µ n
q N (v GS ) = −
•
iD
6.012 Spring 2007
vGS −VT
∫
[vGS − vC (y) − VT
]
2
• dvC
0
Lecture 10
13
Inversion layer charge in saturation
(contd.)
Do integral, substitute iD in saturation and get:
2
qN (vGS ) = − WLCox (v GS − VT )
3
Gate charge:
qG (vGS ) = −qN (v GS ) − Q B, max
Intrinsic gate-to-source capacitance:
Cgs, i =
dq G 2
= WLCox
dv GS 3
Must add overlap capacitance:
2
C gs = WLC ox + WCov
3
Gate-to-drain capacitance — only overlap capacitance:
Cgd = WCov
6.012 Spring 2007
Lecture 10
14
;
;
;
;
Other capacitances
NRS = N (source)
PS = 2 × Ldiff (source) = W
NRD = N (drain)
PD = 2 × Ldiff (drain) = W
L
Ldiff (source)
Ldiff (drain)
W
AS = W × Ldiff (source)
Source-to-Bulk capacitance:
(
AD = W × Ldiff (drain)
)
C sb = WLdiff C j + 2Ldiff + W C jsw
where C j : Bottom Wall at VSB (F / cm 2 )
C jsw : Side Wall at VSB (F / cm)
Drain-to-Bulk capacitance:
(
)
C db = WLdiff C j + 2Ldiff + W C jsw
where C j : Bottom Wall at VDB (F / cm2 )
C jsw : Side Wall at VDB (F / cm)
Gate-to-Bulk capacitance:
Cgb ≡ small parasitic capacitance in most cases (ignore)
6.012 Spring 2007
Lecture 10
15
What did we learn today?
Summary of Key Concepts
High-frequency small-signal equivalent circuit model of
MOSFET
id
Cgd
G
vgs
S
D
+
Cgs
gmvgs
gmbvbs
ro
-
vbs
B
Cgb
Csb
+
Cdb
In saturation:
W
gm ∝
ID
L
L
ro ∝
ID
Cgs ∝ WLCox
6.012 Spring 2007
Lecture 10
16