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Lecture-7
MOS SPICE Models
Dr. Arti Noor,
M. Tech Division, CDAC Noida.
Email : [email protected]
12-10-2009
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What is SPICE?
SPICE is simulation tool for circuits.
Picking of correct SPICE level / model can
save simulation / design time.
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What is modeling?
Future prediction.In this case you are trying to design an IC to
meet a
certain specification.
Many Models to evaluate same thing why?
The more accurate/complex the model the more timeit takes to get
your answer.
Canuse the fastest / less accurate model first to get arough
idea of the solution space of interest.
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SPICE modeling
SPICE stands for Simulation Program with IntegratedCircuit
Emphasis.
Developed at Berkeley.
Latest version is BSIM4.
Hspice is Avantis version.
Spectre is Cadences version for UNIX.
OrCAD is for the PC.
PSPICE is the spice algorithmfor the PC.
But here we use Tanner TSPICE in the lab.
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SPICE modeling (contd.)
SPICE does analog simulations of a digitalcircuits.
The are four process corners FF, SS, FS,
and SF.
We will use extracted parameters from real
process lines.
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SPICE Levels
Level 1 Square-law current voltage.
Level 2
Detailed analytical model. Level 3
Semi empirical model.
BSIM4 sub-micron
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Transient SPICE Model
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SPICE Levels
Level 1
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SPICE Levels
Level 1
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SPICE Levels
Level 1
Thus level 1 is characterize by Five parameters
=
A
iF
N
n
q
kTln22
,2,,, FToVk
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Level-2 Model
This model has the following features :
Does not use GCA. Calculates threshold voltage. Takes variations
of mobility with electric field.
Variation of channel length. Saturation of carrier velocity. Sub
threshold conduction. Level two harder to do by hand.
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Level-2 Model
To calculate current, bulk depletion charges are taken into
account. Current
equation is solved using voltage dependent Bulk charge term:
The equation shows current variation with ,Y, VBS. The
saturation voltage
is calculated as
( )
( ) ( )[ ]
++
=
2/32
2/32
3
2
22
)1(
FBSVFBSVDSV
DSVDSV
FFBVGSV
L
W
DSV
kDI
( )
( )( )DSVDsatIDIFBVGSVFFBVGSVDsatV
=
++=
1
12
21122
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Level-2 Model
Mobility variations: surface mobility deceases with increasing
VG. Mobility
variation is taken into account as
Uc is gate to channel critical field, Ut is gate to channel
critical field variation
due to drain voltage. Ue is exponential fitting parameter. The
channel length
modulation parameter is defined as
( )
eU
dsVtUTVgsVcUoct
ox
siknewk
=
)(
( )[ ]DSVeffL
L
DsatVDSVDsatVDSVAqNSiL
=
++=
2
41
4
2
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Level-2 Model
Velocity saturation parameter : the inversion layer charge for
Vds =Vdsat is
( )
effNAqNsi
DX
DsatVDSVvDX
DXL
vWDsatI
invQ
2
2
2max.
max.
=
+=
=
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Level-3 Model
semi-empirical model.
Uses simpler expressions than MOS2
plus empirical equations to fit experimental
data.
Improves accuracy and reducessimulation time.
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Comparison of Models
Level 1 is not very accurate. It is useful for hand
calculation and rough estimation of circuit
performance.
Level 2 is analytic model and supports varioussecond order
effect. It is very complex model and
takes maximum CPU time.
Level 3 is semi-empirical model. It has sameaccuracy as level 2.
takes less CPU time. It has many
flitting parameters to calculate.
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Next Topic
MOS Inverter : Static Characteristics
(Chapter-5)
