ECE4273
pSPICE analytical exercise #6
version 1.3
MnSUB-1: Analytical assessment of the subthreshold behavior of the nMOST. This exercise is an extension of checkoff
exercise NMOS-1.
Set up the nMOS transistor test circuit as shown by figure nMOSsub-1. It is little more than a simplified form of the
NMOS bias template used for checkoff exercise nMOS-1, except extended to include an extra transistor. Use of VVTs
(component E) is invoked to ensure isolation of one transistor from another.
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M1
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VGS
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M2
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VBS
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VDS
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Figure nMOSsub-1 Circuit template for ID(VGS , VDS) subthreshold assessments for
different device models and/or different technologies
Set all voltage sources to default = 0 except the VDS source, which should be set to 50mV
Parameterize the W and the L of the transistors, with W = {Wx} and L = {Lx}. Let the default values for these dimensions
be 20µm and 4µm, respectively.
For transistor M1, instantiate level-2 parameters into the pSPICE model statement from (http://www.mosis.org/Technical/
Testdata/supertex-scna20-prm.html) and run N71V. And when you do so, change the name of your transistor from
MbreakN to Mn2p.
For transistor M2, instantiate level-49 parameters into the pSPICE model statement from the MOSIS website (http://
www.mosis.org/Technical/Testdata/ami-c5n-prm.html) and run T1AW.. And when you do so, change the name of your
transistor from MbreakN to Mn7p. These parameters are for the BSIM3V3 model. In pSPICE you will have to change
the model level from level-49 to LEVEL=7, and you will have to delete all of the (FORTRAN-77) line continuation prefixes (+) that are at the beginning of each line.
Under transistor M1 change the value for parameter gamma to parametric form {gam1} and under M2 change the value
for parameter K1 to parametric form {K1x}. Under your PARAM call, declare gam1 = 0.5 and K1x = {gam1}. This construct will give you maximum flexibility in assessment of the subthreshold behavior of these two different models relative
to the principal body-effect parameter.
(a) By use of Analysis > Setup > DC Sweep menu, sweep voltage VG Sfrom 0.0V to 4.0V in increments sufficient to
ensure at least 200 pts over this sweep range. Under the Nested Sweep option for the DC sweep, step the voltage VBS
from 0 to -3 in steps of 0.5V and display ID vs VGS. These are the low-field transconductance characteristics.
And, as well, invoke the Analysis > Setup > Parametric option with gam1 stepped from 0.5 to 0.9, setpsize 0.2.
When you run SPICE you will be left with more traces than you probably would like to see, so you should separately display those for transistor M1 and transistor M2. Toggle the ID vs VGS (low-VDS)characteristics for a log10 y-axis display
and measure the low-field subthreshold slope factor n. Do so separately for transistors M1 and M2. As you noted under
checkoff #9, the subthreshold slope factor is the reciprocal of the logarithmic slope for ID vs VGS divided by thermal voltage VT, for ID less than threshold.
Note that traces for the drain currents can be extracted from the menu as ID(M1) and ID(M2), respectively It is not essential that we include a VDC ‘ammeter’ element, in this case.
Take measurements separately for transistor M1 and M2, and compare them to the analytically defined expressions for
subthreshold slope factor: Show comparisons by means of a comparison table, and also by means of a spreadsheet plot.
SPICE level = 2: Physical model
1
1
1
--- = ------------- 1 – --------------------------------------------------------------------------------1+α
n
2
1 + ( 4 ⁄ γ ) ( V GB – V FB ) ( 1 + α )
where V GB = V GS – V BS and V FB = V TO – φ B – γ φ B
SPICE level = 5: EKV Model
1
γ
--- = 1 – -------------------------------------------------------------------------------n
2
2 ( V GB – V TO ) + ( γ ⁄ 2 + φ B )
where V GB = V GS – V BS
SPICE level = 7: BSIM3V3 (and BSIM3V4) model
CD
C it
1
L
L
n = 1 + N factor ----------- + ----------- ( C DSC + C DSCD V DS + C DSCB V BS )  exp  ( – D VT 1 ) ------ + exp  ( – D VT 1 ) ---  + ---------



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C OX C OX
2l t
lt
C OX
where Nfactor, CDSC, CDSCD, CDSCB, CIT, and DVT1 are SPICE level 7parameters, CD = eS/Xdep and most of the rest of the
parameters (such as Xdep and lt) you will have to ascertain on your own from the BSIM3V3 document on the class website
or even the original document at the UCB website (http://www.nikola.com/bsim3v3.htm). The USB hyperlinks for
BSIM3V3 and BSIM3V4 have been added to the class website.
(b) Reset VDS = 4.0V and plot I D vs VGS, same VBS stepsize as before. Toggle the I D characteristics for log10 y-axis
display. These are the high-field characteristics. Measure the high-field subthreshold slope factor n for the transistor.
Show a comparison table and compare to the analytical forms indicated above.
Be compact in your report. But also do not distort the displays so that they cannot be discerned. Negative points.