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inconsistency while measuring sub-threshold currents.

Hi all,

I experience a strange issue when I simulate pmos and nmos devices operating in sub-threshold. The issue is as

follows:

I bias a pmos transistor of minimum channel length and width with ideal voltage sources: VG=VS=VDD, VB=VDD,

VD=0V. I perform a DC analysis and save the DC operating point. Surprisingly, when I annotate the DC operating

points, the ID of the transistor (which is biased in subthreshold) is lower than the DC current than is drawn from

the voltage sources connected at its Source and Drain terminals. The difference is significant, approximately an

order of magnitude or less depending on the bias conditions. This probably means that there is some extra leakage

current through the transistor which I am not able to see via the operating points. This issue is only present when

the transistor is biased in sub-threshold. In moderate and strong inversion, all currents are equal.

I have run this setup in several CMOS technologies, i.e. IBM 130nm, TSMC 0.35um, TSMC 90nm (all of them are

modeled with BSIM4.x). IC and Spectre versions are as follows:

IC 6.1.4-646.485 & IC 5.10.41_USR5.90.69 and Spectre 7.2.0 (64 bit).

Has anyone of you experienced a similar issue? Do you believe this inconsistency between current indications

might be due to modeling issues? Where does the extra leakage current come from, since both p-n junction diodes

are reverse-biased?

Thank you.

Parents

geovol over 9 years ago

Hi Andrew,

Gmin is set to 1e-12 S. Therefore for Vbd=3.2V-3.15V=50mV, idb is equal to idb=1e-12A/V*50mV=50fA, not 3.2pA. You can check this value in the attached picture above.

Decreasing Gmin to 1e-15, all currents agree, i.e. |id|=|ide|=|ise|=112fA.

Do you believe it is realistic to simulate my design for calculating leakage currents in the order of fA with Gmin=1e-15S ?

Thank you,

George.

ps. input.scs file follows:

// Generated for: spectre
// Generated on: Jul 19 11:03:46 2016
// Design library name: pmos
// Design cell name: pmos
// Design view name: schematic
simulator lang=spectre
global 0
parameters vbp=3.2 vsp=3.2 vdp=3.15 vgp=3.3 l=400n m=1 nf=1 wf=500n
include "/proj/cad/library/mosis/IBM_PDK/cmrf8sf/V1.8.0.4DM/Spectre/models/design.scs"
include "/proj/cad/library/mosis/IBM_PDK/cmrf8sf/V1.8.0.4DM/Spectre/models/allModels.scs" section=tt
include "/proj/txace/axa152131/gb130/saveop.scs"

// Library name: pmos
// Cell name: pmos
// View name: schematic
V16 (net023 0) vsource dc=vsp type=dc
V12 (net027 0) vsource dc=vbp type=dc
V11 (net030 0) vsource dc=vgp type=dc
V10 (net022 0) vsource dc=vdp type=dc
I7 (net022 net030 net023 net027) pfet33 l=l w=wf nf=nf m=1 par=1 ngcon=1 \
        ad=(wf*6e-07) as=(wf*6e-07) pd=(2*wf+2*6e-07) ps=(2*wf+2*6e-07) \
        nrd=2.2e-07/wf nrs=2.2e-07/wf rf_rsub=1 plnest=-1 plorient=-1 \
        pld200=-1 pwd100=-1 lstis=1 lnws=0 rgatemod=0 rbodymod=0 panw1=0p \
        panw2=0p panw3=0p panw4=0p panw5=0p panw6=0p panw7=0p panw8=0p \
        panw9=0p panw10=0p sa=6e-07 sb=6e-07 sd=0u dtemp=0
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=25.0 \
    tnom=25 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
    digits=5 cols=80 pivrel=1e-3 sensfile="../psf/sens.output" \
    checklimitdest=psf
dcOp dc write="spectre.dc" maxiters=150 maxsteps=10000 annotate=status
dcOpInfo info what=oppoint where=rawfile
dc dc param=vgp start=0 stop=3.3 step=0.01 oppoint=rawfile maxiters=150 \
    maxsteps=10000 annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
save I7:s I7:d
saveOptions options save=allpub
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geovol over 9 years ago

Hi Andrew,

Gmin is set to 1e-12 S. Therefore for Vbd=3.2V-3.15V=50mV, idb is equal to idb=1e-12A/V*50mV=50fA, not 3.2pA. You can check this value in the attached picture above.

Decreasing Gmin to 1e-15, all currents agree, i.e. |id|=|ide|=|ise|=112fA.

Do you believe it is realistic to simulate my design for calculating leakage currents in the order of fA with Gmin=1e-15S ?

Thank you,

George.

ps. input.scs file follows:

// Generated for: spectre
// Generated on: Jul 19 11:03:46 2016
// Design library name: pmos
// Design cell name: pmos
// Design view name: schematic
simulator lang=spectre
global 0
parameters vbp=3.2 vsp=3.2 vdp=3.15 vgp=3.3 l=400n m=1 nf=1 wf=500n
include "/proj/cad/library/mosis/IBM_PDK/cmrf8sf/V1.8.0.4DM/Spectre/models/design.scs"
include "/proj/cad/library/mosis/IBM_PDK/cmrf8sf/V1.8.0.4DM/Spectre/models/allModels.scs" section=tt
include "/proj/txace/axa152131/gb130/saveop.scs"

// Library name: pmos
// Cell name: pmos
// View name: schematic
V16 (net023 0) vsource dc=vsp type=dc
V12 (net027 0) vsource dc=vbp type=dc
V11 (net030 0) vsource dc=vgp type=dc
V10 (net022 0) vsource dc=vdp type=dc
I7 (net022 net030 net023 net027) pfet33 l=l w=wf nf=nf m=1 par=1 ngcon=1 \
        ad=(wf*6e-07) as=(wf*6e-07) pd=(2*wf+2*6e-07) ps=(2*wf+2*6e-07) \
        nrd=2.2e-07/wf nrs=2.2e-07/wf rf_rsub=1 plnest=-1 plorient=-1 \
        pld200=-1 pwd100=-1 lstis=1 lnws=0 rgatemod=0 rbodymod=0 panw1=0p \
        panw2=0p panw3=0p panw4=0p panw5=0p panw6=0p panw7=0p panw8=0p \
        panw9=0p panw10=0p sa=6e-07 sb=6e-07 sd=0u dtemp=0
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=25.0 \
    tnom=25 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
    digits=5 cols=80 pivrel=1e-3 sensfile="../psf/sens.output" \
    checklimitdest=psf
dcOp dc write="spectre.dc" maxiters=150 maxsteps=10000 annotate=status
dcOpInfo info what=oppoint where=rawfile
dc dc param=vgp start=0 stop=3.3 step=0.01 oppoint=rawfile maxiters=150 \
    maxsteps=10000 annotate=status
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
save I7:s I7:d
saveOptions options save=allpub
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