Parasitic capacitances from printed DC operating point of Spectre (with BSIM3v3) 

Hi all,

I have a  question to ask about BSIM model, I hope it will not take too much time for you to answer this question. Since now I am using a BSIM3.3 0.18-um CMOS model in my work (Cadence, Spectre) and I am wondering about how to figure out the total parasitic capacitances of a transistor. In PSPICE, after I ran a simulation and click on the Probe window to see the output file, I can see a list of small-signal parameters saying that, for example, Cgs = 10 fF (for intrinsic), Cgsov = 20 fF (for overlap capacitance), etc.

However, for BSIM3.3 and for Spectre simulator, I received a set of small-signal parameters after running a simulation, for example, as shown in the attached file. For these parameters, for the capacitances, I don't know how to calculate the total intrinsic and extrinsic capacitances since they are separated into several capacitances. Would you mind to explain me a little bit about this  ?

The complete set of spectre BSIM3v3 output operating point parameters can be found in the "Virtuoso Simulator Circuit Components and Device Models Manual"

http://sourcelink.cadence.com/docs/files/Release_Info/Docs/spectremod/spectremod7.1/chap21.html#133770

 After performing a DC operating point simulation and saving the results, you can use the results browser to select the dcOpInfo (or use print) and browse the available operating point information for a selected component. In my case, I do see "cgs" and "cgsovl" for BSIM3v3

Jim

 Hi Jim,

Thank you very much for the information. I am having a look at the document you mentioned. However, it seems that I am not being able to figure out how to calculate the real intrinsic Cgs, for example. Did you really see Cgs, Cgsov parameters like this from Results --> Print --> DC operating point ?? For my case, as shown in the attached picture, my Cgs is negative and there are also Cgg, Csg, Cdd .., etc. So from this output data, I am very confused about how to calculate the effective value of, for example, intrinsic Cgs.

Would you mind to suggest again :)

 WD

 Hi,

You can refer to these links for explanations of the capacitances in BSIM model:

http://www.edaboard.com/viewtopic.php?t=317385&highlight=negative+cgs

http://www.edaboard.com/viewtopic.php?t=112017&highlight=negative+cgs

http://www.dianzichan.com/anonymous/ic/SpectreVeiledning.pdf 

 Mr Wave...

What version of spectre/MMSIM are you using? I am using spectre 7.1 (a fairly recent version) and get the following from Results --> Print --> DC operating point

 signal            OP("/I0/NM10" "??")

betaeff             282.703m
cbb                  50.9655f
cbd                 -77.4215a
cbdbi                36.8578f
cbg                 -24.7584f
cbs                 -26.1297f
cbsbi                48.289f
cdb                  -4.34466a
cdd                  36.929f
cddbi               -36.9323f
cdg                 -36.9404f
cds                  15.8045a
cgb                 -24.097f
cgbovl              265.891a
cgd                 -36.5642f
cgdovl               36.926f
cgg                 186.958f
cgs                -126.297f
cgsovl               36.926f
cjd                  36.9353f
cjs                  74.4187f
csb                 -26.8642f
csd                -287.387a
csg                -125.259f
css                 152.411f
gbd                  61.5137u
...etc.

Jim

 Hi tkhan,

 Thank you for the information. Actually, I already knew the meaning of negative capacitances. However, what I have never known is how to calculate the "effecitve" value of the capaictances. For example, in case that I would like to know the total intrinsic gate-source capacitance from the operating-point results, how could I do that ??? There are Cgs, Csg, Css, Cgg ... etc. How could I calculate the total Cgs using these parameters ???

 By the way, thank you very much again,

 DW

 Hi Jim,

 Thank you for the reply and for the data. Yes, I also have something look like this from my operating point list. However, I am curious about how to calculate the total, for example, Cgs. As you can see from your op list, there are Cgs (which I am not sure that if it is the total intrinsic Cgs), Csg, Cgg, Css ... etc. Cgsovl seems to be reasonable to be the overlap capacitance between the gate and the source. However, what about Cgs ???

 Thank you very much for your reply again,

 DW

Hi DW

Cgs, Cgd, C[i,j] etc. are the total intrinsic capacitances. See the chapter 4 of the specific version of the BSIM model you are using for the charge equations. This chapter also should describe the extrinsic capacitances such as gate overlap capacitance.

http://www-device.eecs.berkeley.edu/~bsim3/get.html

Jim

 Hi Jim,

Thank you for your information. I have downloaded the document and checked it. Yes, Ci,j in the document means the intrinsic capacitance of each terminal corresponding to the equation Ci,j = dQi/dVj. However, if we would like to calculate, for example, the total intrinsic capacitance between the gate and the source and what we can find from the operating point result are Cgs, Csg, Cgg and Css. I am still wondering how to use these values in the calculation of Cgs_tot ... I am not sure if I can use the absolute value of Cgs or Csg directly (there values even not the same). Then, what about Cgg and Css ... I have not found any document stating about this calculation. But I am still digging into it.

 Thank you for your reply again,

 DW

 Hi,

As far as I know, that is why there is a captab option in spectre. It can be used together with dc simulation or transient simulation at certain points.

It gives you fairly, what you want...

Cheers,

 hi everybody....i want to calculate the node capacitance of digital circuit(eg.adder cmos), and i have done "captab" analysis in spectre.....which gave the table listing capacitance of node, node to ground,node to node of every drain,source,gate terminal....but i am confused whether it is total capacitance(diffusion,intrinsic,extrinsic,node capacitanc due to other)....   .