ltspice -- body effect and other matters

LTSpice support seven different MOSFET models.

Your best plan is to read the appropriate section of the manual, then, if necessary, look up the references cited there.

IME, mosfet models are a minefield. There was an old thread in here where Jim Thompson and Winfield Hill discussed this very subject in detail, in an attempt to model low level behavior. It got very involved. There may be some material relating to this on Jim's website.

Gamma is supprted in the level 1 through level 3 models, BTW.

ALL levels of CMOS models attempt (fairly well) to model "body effect"

The inaccuracies happen at "sub-threshold".

PSpice has a Level=7 model (BSIM3v3) that works well. And I'm sure LTspice has an equivalent.

The problem is that discrete device manufacturers provide only lower level models, typically Level=3.

I do vaguely recall showing that one could fit a Level=7 model to a

I keep my "Sent" directory trimmed at 2 years, so I can't find the pertinent prior posts.

Perhaps someone can find them via GoogleGroups? ...Jim Thompson

ISTR that some stuff was PDFs posted to a.b.s.e, so they won't have those. I did save copies for a while, but I'm not interested enough in mosfets to have kept them indefinitely.

Trawling around here for something unrelated, I found this, apparently originating from Win Hill, dated march 2005:

.SUBCKT 2N7000/PLP 1 2 3 Cgs 2 3 12.3E-12 Cgd1 2 4 27.4E-12 Cgd2 1 4 6E-12 M1 1 2 3 3 MOST1 M2 4 2 1 3 MOST2 D1 3 1 Dbody .MODEL MOST1 NMOS(Level=3 Kp=20.78u W=9.7m L=2u Rs=20m Vto=2 Rd=1.186) .MODEL MOST2 NMOS(VTO=-4.73 Kp=20.78u W=9.7m L=2u Rs=20m) .MODEL Dbody D(Is=125f N=1.023 Rs=1.281 Ikf=18.01 Cjo=46.3p M=.3423

• Vj=.4519 Bv=60 Ibv=10u Tt=161.6n) .ENDS

I did find a simulation comparison on EIGHT different 2N7000 models, only a few came close at sub-threshold.

I'll look for the PDF's, and post a link. ...Jim Thompson

Here you go...

The Supertex model is fairly good, though not perfect, note the "kink". ...Jim Thompson

When I was searching for this file I ran across an IRF120 test that shows that even the Level=3 model properly show the gate charge plateau.

So would could muse if there's a way to take the existing Level=3 stuff and easily convert to Level=7 equivalents, then see what one needs to twiddle to match sub-threshold. ...Jim Thompson

Thanks. (though it was Michael Robinson who originally asked).

The Supertex model certainly looks better than the rest.

When I get the opportunity, I shall have to curvetrace a real 7000.

Do you have a curve tracer than can output numerical data to a file?

If so, send to me and I'll try my hand at fitting it. ...Jim Thompson

No, just trusty old Tek 576, 176, 577.

Graticule, eyeball, pencil and paper :-)