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inconsistency while measuring sub-threshold currents.

geovol

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
  • Andrew Beckett

    You're using a version of the simulator that is at least 6 years old. If there was a bug in bsim4, it's highly likely that it's been fixed by now if it is as apparent as you say. You say there's an order of magnitude difference, but don't see what sort of magnitude the currents are (if they're very small, there's a chance you're seeing numerical noise).

    Without specific details of the problem (ideally a test case that shows the problem) it's hard to comment further, so I suggest that if the problem still occurs once you've moved to a simulator version from this decade that you contact customer support providing the details of the problem (the circuit, simulation setup, and precise technology reference that you're using). There's also a potential problem with how the models have been implemented, although it seems unlikely that you'd see the same problem in multiple technologies if this is the reason.

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    You're using a version of the simulator that is at least 6 years old. If there was a bug in bsim4, it's highly likely that it's been fixed by now if it is as apparent as you say. You say there's an order of magnitude difference, but don't see what sort of magnitude the currents are (if they're very small, there's a chance you're seeing numerical noise).

    Without specific details of the problem (ideally a test case that shows the problem) it's hard to comment further, so I suggest that if the problem still occurs once you've moved to a simulator version from this decade that you contact customer support providing the details of the problem (the circuit, simulation setup, and precise technology reference that you're using). There's also a potential problem with how the models have been implemented, although it seems unlikely that you'd see the same problem in multiple technologies if this is the reason.

    Regards,

    Andrew.

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