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Modeling >> Semiconductor Devices >> NQS behavior
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Message started by Paul on Oct 6^th, 2003, 5:42am

Title: NQS behavior
Post by Paul on Oct 6^th, 2003, 5:42am

Hi all,

I just read some stuff about the non-quasi static models for MOSFETs at high frequencies. I wondered if anybody has some experience with these. What about the quality of the model parameters? For a state-of-the-art technology (0.18u, 0.13u), what frequencies should I start using these models (1GHz, 10GHz,...)?

Thanks for your comments

Paul

Title: Re: NQS behavior
Post by skippy on Oct 12^th, 2003, 7:44pm

Do you have any specific references? I've seen some papers where they state the Quasi-Static models are accurate to some fraction of the cutoff frequency (e.g. 1/12). Some NQS models work by trying to solve an additional internal continuity equation or by segmenting the transistor into smaller transistors.

Is this a digital or analog application? Analog applications may require more accuracy if the nonlinear mixing behavior is important.

Title: Re: NQS behavior
Post by Paul on Oct 12^th, 2003, 11:29pm

In fact I attended a course about high-speed design and the NQS subject was considered too. My application is analog, in the 1-5 GHz range. I wonder in how far you can just set the NQS switch in the model and whether the modeling people in the foundries did check the model in NQS state.

Using the models in the technology we use for our design and simulating both QS and NQS cases I only see differences above 20 GHz. But as mentioned before, I don't know if the model parameters are still valid for the NQS case.

Paul

Title: Re: NQS behavior
Post by skippy on Oct 13^th, 2003, 9:51pm

I am not really a compact modelling kind of guy. Tsividis has a whole chapter on large-signal modelling and discusses the QS and NQS effects. He has another chapter on high-frequency small-signal models where he also discusses NQS effects, noise, and MOSFET modeling for RF applications. Overall a great MOS book.

Yannis Tsividis, Operation and modeling of the MOS transistor, 2ed, 1999.

Title: Re: NQS behavior
Post by Paul on Oct 14^th, 2003, 11:49pm

Thanks for the reference!

Paul

Title: Re: NQS behavior
Post by EVI on Dec 1^st, 2003, 12:39pm

Hi Paul,

You can see a noticeable difference between QS and NQS mode at much lower frequencies if you try to simulate charge injection effects in switched-cap circuits. I tried this but I'm still not sure if I can trust the accuracy of chrge injection simulations even with NQS model.

Eugene.

Title: Re: NQS behavior
Post by Paul on Dec 1^st, 2003, 11:16pm

Thanks for the hint, Eugene. I never tried NQS simulation with switched cap circuits. I don't trust the simulated channel charge too much, so I prefer to rely on cancelling charge injection (using fully diff circuits etc) to estimating the degradation due to it.

Do you think the model parameters are still valid for NQS operation, as probably the model has not been characterized for this mode. The simulated behavior is different from the QS results, but does it correspond to the real behavior of the device?

Paul

Title: Re: NQS behavior
Post by Geoffrey_Coram on Feb 13^th, 2004, 9:53am

My recollection is that BSIM3 has a parameter, the Elmore delay, that is used only for NQS. If you don't have that in your model card, then you probably can't trust the NQS results.

BSIM4 uses one of the gate resistances plus the oxide capacitance to set the time constant; the gate resistance of your model may or may not have been partitioned correctly to get the right NQS result.

Title: Re: NQS behavior
Post by Paul on Feb 16^th, 2004, 12:16am

Geoffrey,

thanks for this interesting point. My BSIM3 models don't have the Elmore delay, i.e. the simulator is using the default value. So I guess the NQS model parameters have not been checked.

Paul

Title: Re: NQS behavior
Post by erichih on Apr 20^th, 2006, 11:23am

I happen to study something about NQS recently. Hope this will help.

NQS effect happens when input frequency is pretty high at the order of transit frequency. A resistor, which is series connected with the Cgs, is used to model the time delay between the stimulus at the gate and the response at the channel by Elmore, and the resistance value can be expressed by 1/(5*gm). Then later on, Prof. Chenming Hu published some papers, saying that the NQS effect could be best modeled by an external resistor, whose value is extracted from the y parameters. So even there are no ELM or NQSMOD parameters in your model, which is QS model. You still can use an external resistor to simulate the NQS effect. Moreover, the inclusion of an external resistor at the gate termal can give us another benefit, i.e. the gate-current noise, which is correlated to MOSFET thermal noise, through Cgs capacitive-coupling. (Why? Please go to see Prof. Thomas Lee's paper about 1.5GHz LNA.) I believe that this kind of noise is not included in the model, neither.

---Eric

Title: Re: NQS behavior
Post by Geoffrey_Coram on Apr 25^th, 2006, 5:26am

*Some* NQS effects can be captured by adding an external gate resistance, but not all. Philips uses a channel-segmentation model, and I've heard they have a subcircuit model that automatically computes the parameters for the segments from the original unsegmented model.

I'd also recommend looking at the papers on NQS in the PSP model, look for papers by Hailing Wang and G. Gildenblat from Penn State.

Title: Re: NQS behavior
Post by 337see733 on Apr 27^th, 2006, 9:29pm

The NQS effects are important when transit delay is comparable to the device rise/fall time. So, this become important either of these two conditions:
1.)Long channel device
2.)High frequency

For analog, either of these conditions usually apply, it is advisable to include NQS effect in the simulation.