sheldon
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Arlo,
The question is complex. The SPICE Gummel-Poon model does not scale well because the scaling assumptions are primitive. The scaling basically assumes a fixed emitter length with no modeling of end effects. In addition, the model has no capability for distinguishing between bipolar transistor structures, for example, modeling the effect of double base contacts on base resistance. I think that some of the more modern models do provide this capability, HiCUM(?). In practice, I have seen both scalable and fixed geometry based bipolar models. For scalable Gummel-Poon modeling, the model parameters are defined as functions of geometry. This actually works very well and can be combined with statsitical modeling, that is, scalable bipolar models with statistical dependency. See the Spectre Monte Carlo modeling application note for some discussion of how to include geometric dependency in models. The only problem with this approach is finding someone who is willing to do the modeling! On the other hand, using fixed models is a simple and robust methodology that the fab tends to prefer. These comments apply to vertical devices. I don't think that scaling of lateral devices is appropriate other than using multiple devices. Also, AC is correct about modeling, it is better to use scale factor rather than scaling emitter area if you are interested in model accuracy, however, you will pay a price in performance. For the effect of parasitics tends scale down as the device scales up so 10x larger device will have better speed-power performance than 10 1x devices.
Best Regards,
Sheldon
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