[Magic-dev] Correct perimeter values for mosfet diffusions in spice

[Graham Petley]

   This isn't specifically Magic, but its extractor works in a
certain way and I want to be certain it is correct.

   What perimeter should be included in the PD and PS values used
on mosfet cards in a spice deck?

   If we take programs like Magic, Alliance and Electric, then the
perimeter extracted is the length of the diffusion to field PLUS the
width of any transistors connected to that diffusion.

   But if I remember back to the days when we had to manually measure
the perimeter (we're talking about the '70s here), then the width
of the transistors was excluded from the PD/PS calculation.
   This is also the instruction in this paper I found on the web.
Look at slide 4.

http://users.ece.gatech.edu/~rincon/classes/ece3040/handouts/spice3.pdf

   So which method is correct?
   I think excluding the transistor width from the calculation should
be correct because
- spice knows the transistor width anyway from the circuit description
  so there's no need to add it in a second time as part of the
  perimeter;
- the diode cap coeff to the diffusion under the gate can be different
  to the bulk junction coeff.

best regards, Graham Petley