Hi <!here> M vs NF. We are creating a design and we are working now on finishing some of the layout and when I looked closely to the netlist from xscheme I saw something like this:

XM6 Vout net2 VDD VDD sky130_fd_pr__pfet_g5v0d10v5 L=0.5 W=20 nf=1 ad='int((nf+1)/2) * W/nf * 0.29' as='int((nf+2)/2) * W/nf * 0.29'
+ pd='2*int((nf+1)/2) * (W/nf + 0.29)' ps='2*int((nf+2)/2) * (W/nf + 0.29)' nrd='0.29 / W' nrs='0.29 / W'
+ sa=0 sb=0 sd=0 mult=20 m=20

nf = 1, mult=20 and m = 20. I'm a little bit confused because I'm not sure if the model will take source/drain sharing correctly in that case? Nor does it takes the LOD effects.

As far as the schematic goes, "nf = 1, M= 20" implies 20 transistors not sharing any source/drain area. As far as magic goes, it is a known issue in magic that it does not recognize or output device fingers, mainly because the extraction method long pre-dates the BSIM model. I am hoping to fix this along with other issues related to parasitic extraction in the near future.

@Tim Edwards Oh, I see. Great to know.

I don't get that. Xschem just outputs whatever you tell it to. If you want fingered devices, then you set W and nf appropriately.

The above is from Xschem.

I'm not sure what you're doing in xschem, but if you want, e.g.,

XM6

from your example above to be properly represented in xschem, you would want to set

W=20

nf=20

m=1

mult=1

.

W=400 ?

@Amro Tork ok

No. According to the BSIM definition,

W

is the total width of all fingers.

@Tim Edwards agree.

Unfortunately, the ngspice developers have decided that interpreting BSIM models according to the BSIM definition should not be the default behavior, so make sure you are using the latest version of ngspice and the

spinit

file that is distributed with open_pdks and sets the right behavior for correctly interpreting

W

,

nf

, and

M

.

@Tim Edwards path to the "spinit"?

And yes, sorry, to match the example you would want

W=400

. I was not sure of the intent of the example.

@Eslam Morsie Could you please make sure we have those switches set properly in our models?