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Chris Jones

12/05/2020, 3:17 PM
@User and anyone else who can help: Sorry for the rather basic question, but what is the correct way to specify (xschem) a mosfet with multiple fingers? Is there a specific version/commit of ngspice or the pdk that I would need in order for it to simulate properly? For example I can place a nfet_01v8 with L=0.15 and W=5, and that seems to do what I would expect, but if I wanted a device with 10 poly gates the same size as that one so a total width of 50um, and therefore 5 drain diffusions, and 6 source diffusions, I can't figure out how to achieve this. I tried setting W=50 and nf=10 but this seems to just make ngspice not converge (for a very simple circuit that just sweeps the gate voltage and plots Id) - I was wondering if it fails to find the bin for W=5 in this case. Setting W=5 and mult=10 seems to give the right DC results for a 50um device, but then the drain capacitance goes up 10x whereas I think it should be only 5x higher because two poly gates share each stripe of drain diffusion. I guess I could edit the expressions for ad and pd but I hope it shouldn't be necessary to do this by hand for the standard layout of a mosfet with several fingers. I used yrrapt's fork of sky130_setup to install ngspice.