257
MOSFET Capacitance Models
where:
C jbs is the unit-area bottom source-body junction capacitance
C jbssw is the unit-length source-body junction sidewall capacitance along
the isolation edge
C jbsswg is the unit-length source-body junction sidewall capacitance along
the gate edge
A seff and P seff , are the effective junction area and perimeter, on the sourceside of the S/D diffusion, respectively
W effcj and NF are the effective width of the S/D diffusions and the number
of fingers, respectively
The components of the S/D pn-junction capacitances are obtained using the
following expressions:
• C jbs is calculated by
C
CJS T
V
PBS T
V
CJS T
MJS
V
PBS
jbs
bs
MJS
bs
bs
=
⋅ −






<
⋅ +
−
( )
( )
( )
(
1
0
1
,
T T
V bs
)






≥







,
0
(6.108)
• C jbssw is calculated by
C
CJSWS T
V
PBSWS T
V
CJSWS T
jbssw
bs
MJSWS
bs
=
⋅ −






<
⋅ +
−
( )
( )
,
( )
1
0
1 M MJSWS
V
PBSWS T
V
bs
bs
( )
,






≥







0
(6.109)
• C jbsswg is calculated by
C
CJSWGS T
V
PBS GS T
V
CJSWGS
jbsswg
bs
MJSWGS
bs
=
−






<
−
( ).
( )
,
(
1
0
W
T T
MJSWGS
V
PBS GS T
V
bs
bs
).
( )
,
1
0
+






≥







W
(6.110)
6.6.2 Drain-Body Junction Diode
The drain-side pn-junction capacitance can be calculated by
C
A C
P C
W NF C
bd
deff jbd
d eff jbdsw
effcj
j bdswg
=
+
+
⋅
(6.111)