@User @User I have verified that: 1. If a (subcircuit) MOS instance in the netlist does not have any AS,AD, PS, PD parameters the MOS inside the subcircuit wrapper gets all parameters set to zero. Ngspice has a 'listing expand' command that reports the netlist after circuit expansion and parameter substitution, and it confirms this:

14685 : m.xm2.msky130_fd_pr__nfet_01v8 net2 g net6 net4 xm2:sky130_fd_pr__nfet_01v8__model l=    1.49999999999999994e-01   w=    1.00000000000000000e+00   nf=    1.00000000000000000e+00   ad=    0.00000000000000000e+00   as=    0.00000000000000000e+00   pd=    0.00000000000000000e+00   ps=    0.00000000000000000e+00   nrd=    0.00000000000000000e+00   nrs=    0.00000000000000000e+00   sa=    0.00000000000000000e+00   sb=    0.00000000000000000e+00   sd=    0.00000000000000000e+00   m=    1.00000000000000000e+00

1. However having these parameters set to zero does NOT mean there are no drain/source to substrate junction diodes, diodes clamping forward bias conditions still exists, however the I/V characteristics are different. 2. Transistor with no area/perimeters set has lower substrate leakage, picture in following post 3. the IV characteristic of the Body terminal when forward biased w.r.t source and drain (set at 0V) is strange: at 27C transistor with no area/perimeters set has higher current, while at 150C it has lower current. I do not understand the reason.