LTspice – Transistor Model Issue

ltspicespicetransistors

I am importing a custom transistor model in LTspice. Simulating the family of curves for a single device works perfectly though adding another transistor causes the simulator to go incredibly slow and not finish.

For an example I am trying an NMOS inverter circuit, with a DC simulation varying the input gate voltage from -3 to 10VDC in 1V steps. If I hover over the top left of the window I can see text at the bottom reading: "Stepping source: 100% step size =2.95928e-005 N-R iterations: 39 fill-ins:0" while the text changes the iterations.

Here is the model I am using:

.MODEL PlanarFET NMOS 
 + LEVEL = 3 
 + L = 2E-06 
 + W = 1E-05 
 + UO = 9.67 
 + VTO = 1.5 
 + NFS = 0 
 + TOX = 3E-08 
 + NSUB = 1.616E+16 
 + NSS = 0 
 + VMAX = 2183 
 + RS = 4.441E-11 
 + RD = 3.09E+04 
 + RSH = 0.03236 
 + CBD = 0 
 + CBS = 0 
 + CJ = 0 
 + MJ = 0.5 
 + CJSW = 0 
 + MJSW = 0.33 
 + IS = 4.277E-08 
 + PB = 0.8 
 + FC = 0.5 
 + XJ = 0 
 + LD = 0 
 + DELTA = 0 
 + THETA = 0 
 + ETA = 0.4576 
 + KAPPA = 0.1077 

L= 2u
W = 10u
AD = 9p 
AS = 9p
PD = 12u
PS = 12u

LTspice simulation file: https://easyupload.io/heso8u

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I have tried with the default spice simulator settings, and also changing the setting without luck. Any suggestions?

Best Answer

First, it will do you good to draw more readable schematics -- but also for the rest of us, trying to read it, peacefully.

Then, the level=1,2,3 MOSFETs are really old models from the 80s, level=1 dating back to the original SPICE, some 50 years ago. And they have their own problems, one of them being rather stiff and not very friendly, derivative-wise. They will work but, trying to build on them may fail, from time to time. As @jonk notes in the comments, the BSIM models (level=8 and above, see the help under LTspice > Circuit Elements > M. MOSFET) are better behaved and sometimes it pays to allocate a few more minutes to convert the parameters.

To help with convergence, Mike (the creator of the program) said that it is better to use current sources terminated into appropriate resistors (also see the help under LTspice > Circuit Elements > E. ..., last paragraph), or adding some series resistance to the stiff voltage sources. For this case, however, the alternate solver is enough. It's meant to be about 1000x more precise than the normal solver but, at some 2x slower speeds. It depends, though: for some circuits it's actually faster but, then, those circuits would have crawled under the normal solver. When in doubt, try it, that's why the option is there. Most often than not, the alternate solver will get you over many glitches.

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