MOSFET PSP model usage of NF parameter

[hpretl]

When I do a simple testbench in ngspice simulating just a single LV-NMOS (I assume it is the same for PMOS and HV variants, but I have not checked it), I see that the model parameter rg is not reacting to the number of fingers I set. This is not ideal, as the PSP model can model extrinsic resistances.

From the PSP model doc:

The calculation of rg is:

In the model file of IHP's PDK, the PSP model is instantiated like that:

This means that essentially the nf (number of fingers, here called ng, which is not consistent) parameter is never passed on to the PSP model, instead only the width of a single finger is used in the model, and the multiplier parameter m is then increased by nf as passed on to the model as mult=ng*m. In this way, the intrinsic model does not know about the true nf and m parameters. This is also confirmed by looking at the model parameters in ngspice, which always shows nf=1.

Maybe this can be changed in a future release? It looks like some capabilities of the PSP model are bypassed in this way.