Arpad, Personally, I view SPICE as a nonlinear, time variant, ordinary differential equation solver. The techniques used in SPICE are mainly numerical integration (to discretize the time variable), and numerical iteration (to linearize nonlinear devices). Thus, from a solver point of view, the inaccuracies could come from tolerance setting (how tight you want to set a convergence criteria during iteration), and time point selection (how small you want to choose a time step to catch the changing waveforms). Fortunately, in most SPICE tools, the above two are normally set automatically so that users don't need to worry them too much (there are also quite some SPICE books discussing these techniques and how to tune them). Of course, SPICE comes with built-in semiconductor device models (diode, bjt, mosfet, etc). It seems this is the major inaccuracy source we are focusing on. Strictly speaking, a semiconductor device is governed by Poisson's equation and continuity equation, which are both partial differential equations. How to incorporate these equations into SPICE is a formidable task: not only you need to keep the basic physical behavior of the model, but also simplify the model significantly so that it can be evaluated efficiently. To achieve this conflicting goal, many heuristics are developed and employed to make the model valid across a wide range of technologies and geometries. There are many good examples of such model development, the BSIM1-4 model family is a good case. Even after a set of model equations has been developed and verified, it is not over - you need to characterize the model to fit into a specific technology, the process of model parameter extraction. Nowadays a device model can easily have over 100 model parameters, how to extract them deserves a separate topic. Muranyi, Arpad wrote: >Hassan, > >There may be several reasons for inaccuracies in SPICE. > >1) Bad process file. Many times the process files are developed > for the smallest transistor size, and at other sizes (such as > in the larger I/O buffers) they are all over the place. > Not a single set of model equations can fit all technologies. Likewise, not a single set of model parameters can fit all device sizes. That's why a model binning is necessary. Nowadays, almost all foundries distribute their device models in binning. It is a surprise to see people use one set of model parameters for all device sizes. > >2) Larger companies have their own proprietary SPICE tools, > which usually have process parameters which are incompatible > with commercial tools. Converting these process files to > something that commercial tools can do results in loss of > accuracy. Kind of like if you wanted to convert a level=3D13 > MOSFET model to level=3D3. > It is not a good idea to convert one model to another. It could be true that a proprietary model may better fit to its own technology. But if it can not be used to the pulic (especially to its customers), the accuracy advantage can not be fully utilized. > >3) I also hear a lot lately that the small geometries we are > using these days are starting to push the SPICE equations > to the edge where they are not all that accurate or valid > any more. > I agree, developing model is an on-going effort to take more physical effects (gate tunneling, carrier velocity overshoot, to name a few) when the geometry further shinks. Comparing to the old models 10-20 years ago, the latest device model can fit today's dominant technology amazingly well. > >4) For various reasons vendors release reduced "equivalent" > circuits to the customer, which is not as accurate as the > full design. > Developing a useful model is a delicate balance in generality, accuracy, complexity, and efficiency, and demands intimate knowledge of semiconductor physics, process, circuits, and simulation techniques. No matter what format it uses (C, IBIS, AMS), a good model always requires lots of effort and dedication (and lots of resources). Best regards, Yu == > > >There may be more, but I could only remember these right now... > >Arpad >----------------------------------------------------------------- > > >-----Original Message----- >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] = >On Behalf Of Hassan O. Ali >Sent: Thursday, May 26, 2005 2:17 PM >To: si-list@xxxxxxxxxxxxx >Subject: [SI-LIST] Re: SI models at MGH speeds > > >On May 26, "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx> wrote: > >>=20 >>On the other hand, the reason people turn to encrypted SPICE >>models is simply because they do not have the time (or don't >>want to take the time) to figure out how to make behavioral >>models. If IBIS models were hard enough to make, writing >>macro models and *-AMS models are even harder. However, >>SPICE may not be the solution either. For one, it is slow, >>but I also hear issues with its accuracy (contrary to the >>popular belief that it is accurate). >> > >Arpad, > >What did you hear about the source of SPICE errors? Are the errors due = >to the failure of=20 >SPICE to accurately handle multi-GHz SPICE device models or something = >else? > >Thanks. > >Hassan. >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > >List technical documents are available at: > http://www.si-list.org > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu