NGspice 26 returns result that is 6 orders of magnitude too small for simple depletion MOS test jig
Indeed, the gate width is 5.5 meters. Apparently somebody
just twiddled the knobs to get the outcomes he wanted 
(measured). With l and w on the device instantiation 
line the current is 34A @ 4V.

Test of IXTT20N50D model

.model ixtt20n50d nmos (
+ level=3
+ kp=1.0446u
+ rs=1.0000m
+ rd=.22202
+ vto=-.89028
+ tox=2.0000u
+ cgso=3.5684n
+ cgdo=37.622p
+ cbd=4.8729n
+ mj=1.5000
+ pb=2.6055
+ is=1.3714u
* + l=2.0000u
* + w=5.5000
* + rds=20.000MEG
* + rg=10.000m
* + n=2.0283
* + rb=1.0000n
+ gamma=0
+ kappa=0 )

v1 1 0 DC=0
v2 2 0 DC=30
m1 2 1 0 0 ixtt20n50d l=2u w=5.5

.dc v1 -3 4 0.1

.control
  listing e
  run
  plot i(v2)
  write bug60.raw
  *quit
.endc

.end