Models versus Models

I gave a presentation last year at the MOS-AK workshop in San Francisco. The people attending this workshop are all interested in simulation models for semiconductors. One of the comments made during the workshop, was that circuit designers blame the models when their circuits do not work right. In most cases what is meant as model, is really the model parameters that are created by a foundry for a specific simulation model.

The real models go by names such as BSIM, PSP, Gummel-Poon, HICUM. These models are the sets of equations that mimic the electrical behavior of a device. The model is of itself useless without a set of parameters that are process and geometry unique. These parameters are created (fitted) from test circuits the foundry creates. A circuit simulator implements the particular models equations, this coupled with the model parameters, and a netlist describing the desired circuit will give a simulation of how the circuit should work.

A great deal of work goes into the model. This work is reviewed by many people. While mistakes can occur in the model, because of their wide adoption and peer review they tend not to be buggy. The model parameters on the other hand do not experience the same peer review process. The methods of extracting data to fit to the model may vary greatly, how well the data is fitted to the model is variable. Some foundries have very accurate model parameters, some not so accurate. When circuit designers talk of model problems most often they mean model parameter problems.

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