how to determine the subthreshold process paramter?

Not sure where that equation comes from - you need to remember that any such equation may only reflect a specific model, and most modern transistor models are rather more complex than this (although it doesn't look like any basic transistor models either to me, but it may just be how it's written).

I could see that many of the "Philips" (now NXP) models (e.g. MOS11, MOS902 etc) have a parameter for subthreshold slope (this is a model parameter, called "mo" or "mor"), but there doesn't seem to be anything like that for bsim3v3, bsim4. So I suspect the model formulation doesn't have any similar parameter and it's not something the models compute and spit out either (I've not found any requests for this as an output or operating point parameter).

So given that you didn't say which model you're using, or which simulator you're using, or which version of the tools you're using, I rather doubt this can be answered. However, even knowing that, it doesn't look to me as if the equation you've described matches any of the models (from a very quick scan through the model documentation, which is in cdnshelp) in Spectre.

Regards,

Andrew.

Hi Andrew,

Thanks for the reply. I uses spectre mmsim14, virtuoso 616

This is not a particular model. This is a general fact. For a reasonable channel length (Lmin => 500nm), in saturation, strong inversion, the mosfet exhibits square law characteristics. In subthreshold(weak inversion), the mosfet exhibits exponential characteristics like a BJT, with the addition of "nonideality factor" or "slope factor".

If we only look at the the exponential term:

BJT: exp{ [Vgs-VT] / [kT/q]  }

MOSFET: exp{ [Vgs-VT] / [n*(kT/q)]  }

This additional n term is typically around 1~1.5

In the slide 15 in the below link, n is defined as:

n = 1 + Cdep/Cox

Which seems like something that would vary with operating point. Maybe I could try to Cdep and Cox in the list of operating point parameter??

 Elad Alon's notes below:

http://bwrcs.eecs.berkeley.edu/Classes/icdesign/ee240_sp10/lectures/Lecture03_MOS_Models_2up.pdf

I'm aware of the fact that gm/Id dominates over the square law and the weak inversion model I described, but they do give design insights, and are still extensively used in papers in reference generators.

Thank you,

Menghan

Unknown said:

This is not a particular model. This is a general fact

No, it's a model. All equations describing MOSFET behaviour are models which are an attempt to fit the physical behaviour to a certain level. The precise equations will vary from one model to another. Even in the slide you refer to, it's described as a model.

The fact that you're using a particular model to gain design insight is fair enough, and most of us do that, but you can't necessarily expect a simulator to output a parameter which works for the particular model you're using (and is different from the model the simulator is actually using). The models in the simulator will model the subthreshold behaviour you describe, but won't necessarily do it with the equation you're describing (especially as it's incomplete).

Regards,

Andrew.

Hi, I am using Cadence Virtuoso ADE L to simulate my circuit and I want to figure out the subthreshold factor. Can you help me out how to do so?

Arefinn said:

Can you help me out how to do so?

Not unless you both read the thread here (especially the last sentence in my reply to which you posted your question) and can provide a lot more information. What is this "subthreshold factor" you refer to? Which simulator and device models are you using? Which version of the tools are you using? Please provide a reference for any equation you might mention.



and I am using the kit GPDK090

Cadence (R) Virtuoso (R) Spectre (R) Circuit Simulator
Version 13.1.1.117.isr8 64bit -- 19 Jun 2014

You still didn't give a reference for those equations - just an image. In what text are they published? They don't look that familiar to me, and certainly don't match the bsim3v3 model equations used in GPDK090.  Presumably the reference [13] gives some more detail on this sub-threshold slope factor?

Unfortunately most of the relevant text books I have on this are stranded in the office, so I'm having to go on memory, a bit of googling, and looking in the Spectre Circuit Simulator Components and Device Models Reference manual.

Andrew.

The reference is Wang, A., Calhoun, B. H., & Chandrakasan, A. P. (2006). Sub-threshold design for ultra low-power systems. New York: Springer.

https://link.springer.com/content/pdf/10.1007%2F978-0-387-34501-7.pdf

Given that key authors in chapters of that book (I'm not familiar with it, and don't have access to it) are Christian Enz and Eric Vittoz, and it also talks about the EKV model (these two are the E and V in EKV), I would have expected that maybe this equation shows up in the documentation for the EKV or EKV3 model in Spectre. It doesn't. So I don't think I can help you - you'd be trying to fit one equation against another - and given that gpdk090 uses bsim3v3 (which is a totally different approach to compact MOS modelling than EKV), it's going to be difficult.

Andrew.