#74 DC transfer characteristic sim fails at low supply voltages

[Anno]

I am trying to simulate a simple inverter transfer function - DC voltage out & shoot through current vs. DC voltage in.
Using the n-mosfet and p-mosfet symbols provided with the pre-filled in device parameters.
Also tried other Spice Level 1 parameter sets to make sure the as-supplied devices
were not implementing something unphysical that causes convergence problems.

The result is that as I lower the voltage, I first get warnings about DC analysis failures
and fallback solver being deployed, then simulations hang at certain DC steps,
and at even lower voltages the simulations run but produce unphysical results:
the transfer characteristic starts looking like a staircase.

???

Thanks,

Anno

[Anno]

[Anno]

Note that an inverter will switch even below the threshold voltage -
just very slowly, but that is not an issue in DC simulations.

Here is the netlist:

# Qucs 0.0.15 /Users/Anno/.qucs/test_inverter_prj/inverter_rev_a.sch

Vdc:V1 invin gnd U="vin"
.DC:DC1 Temp="26.85" reltol="0.001" abstol="1 pA" vntol="1 uV" saveOPs="no" MaxIter="150" saveAll="no" convHelper="none" Solver="CroutLU"
MOSFET:T1 invin invout gnd gnd Type="nfet" Vt0="1.0 V" Kp="2e-5" Gamma="0.0" Phi="0.6 V" Lambda="0.0" Rd="0.0 Ohm" Rs="0.0 Ohm" Rg="0.0 Ohm" Is="1e-14 A" N="1.0" W="1 um" L="1 um" Ld="0.0" Tox="0.1 um" Cgso="0.0" Cgdo="0.0" Cgbo="0.0" Cbd="0.0 F" Cbs="0.0 F" Pb="0.8 V" Mj="0.5" Fc="0.5" Cjsw="0.0" Mjsw="0.33" Tt="0.0 ps" Nsub="0.0" Nss="0.0" Tpg="1" Uo="600.0" Rsh="0.0" Nrd="1" Nrs="1" Cj="0.0" Js="0.0" Ad="0.0" As="0.0" Pd="0.0 m" Ps="0.0 m" Kf="0.0" Af="1.0" Ffe="1.0" Temp="26.85" Tnom="26.85"
MOSFET:T2 invin vrail invout vrail Type="pfet" Vt0="-1.0 V" Kp="2e-5" Gamma="0.0" Phi="0.6 V" Lambda="0.0" Rd="0.0 Ohm" Rs="0.0 Ohm" Rg="0.0 Ohm" Is="1e-14 A" N="1.0" W="1 um" L="1 um" Ld="0.0" Tox="0.1 um" Cgso="0.0" Cgdo="0.0" Cgbo="0.0" Cbd="0.0 F" Cbs="0.0 F" Pb="0.8 V" Mj="0.5" Fc="0.5" Cjsw="0.0" Mjsw="0.33" Tt="0.0 ps" Nsub="0.0" Nss="0.0" Tpg="1" Uo="600.0" Rsh="0.0" Nrd="1" Nrs="1" Cj="0.0" Js="0.0" Ad="0.0" As="0.0" Pd="0.0 m" Ps="0.0 m" Kf="0.0" Af="1.0" Ffe="1.0" Temp="26.85" Tnom="26.85"
.SW:SW1 Sim="DC1" Type="lin" Param="vin" Start="0" Stop="1.8" Points="361"
Vdc:V2 vrail gnd U="1.8"