#646 Discrepancy between ngspice and hspice in current for low vgs

[Chad Winters]

I am using the freepdk45nm. I simulated the drain current vs. the gate-source voltage. I found discrepancy between the two when the gate-source voltage is very low, probably due to the fact that ngspice does not model the leakage current?

[Chad Winters]

* Test *
.include '/home/medwatt/models/NMOS_VTH.lib'
VGS NG 0 DC=0
VBS NB 0 DC=0
VDS ND 0 DC=0
M1 ND NG 0 NB NMOS_VTH l=6.4e-06 w=1e-05

.options TEMP = 27
.options TNOM = 27
.control
save @m1[id]
dc VDS 0.0 1.0 0.01 VGS 0.0 1.0 0.01
write /tmp/tmps74832zi all
.endc
.end

[Dietmar Warning]

I see a discrepancy between your dc sweep command and the two figures, which should show a length parametric sweep @ 0.5V. You are sure that you picked the 0.5V VDS sweep for ngspice?

The op template @m1[id] reflect not the true drain current for analysis. Use abs(i(vds)) for save, write and plot and everything is OK.

[Chad Winters]

@dwarning, thanks for reply. The issue was due to me using @m1[id]. Using i(vds) fixed the issue.

[Dietmar Warning]

Yes, the op variable [id] is the pure channel current and did not include d/s diode contributions.