Devlog #7

Since my last devlog I have been teaching DEVSIM to run the same physics my own solver runs. Before this it was on a stripped down model set, so the two codes had similar geometry and nothing about mobility. Now it runs the real thing: doping dependent mobility, surface scattering at the oxide, velocity saturation, and proper Fermi-Dirac statistics.

Getting there meant three more small lies.

DEVSIM will easily chain a derivative through one kind of model and silently return zero for another. No error, no warning. My solver just diverged on the first step. It took a dump of every derivative on the device to see a column of exact zeros where numbers should have been.

Then a field magnitude written as a square root of a square. Mathematically, sounds good but numerically it produces a nan wherever the field is exactly zero, which in a neutral chunk of silicon is nearly every edge.

Then the surface mobility. I was refreshing it on every pass, forever. That is not a converging loop, that is two numbers chasing each other. The gate walk climbed to 0.50 V and then refused 0.55 V at every step size I gave it, down to a tenth of a millivolt. A failure that ignores step size is not a step size problem. I gave the refresh a stopping rule and re-ran it. Same answer to the last digit, just reachable now.

The 1 micron device now agrees between the two codes to under 3.3 percent at low drain and under 1.3 percent at high drain, across four decades of current.

The remaining blocker is not code. My browser is holding 15 of the machine’s 16 GB.