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Lack of systematic approach. Frustration. [RANT] (Read 42 times)
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Lack of systematic approach. Frustration. [RANT]
Mar 16th, 2017, 5:31pm
I am trying to perform AlGaN/GaN HEMT 2D simulation up to breakdown voltages (1000 V to 2000 V). I face various convergence failure messages from Sentaurus TCAD. Sometimes it works and sometimes it doesn't.

The problem is: I have not been able to systematically diagnose the cause of convergence failures.

  • It is well-known that we need a high-resolution mesh around the 2DEG. First of all, why? where does this folk knowledge come from and where is the systematic justification for it? You might say, "why don't you try the coarse mesh and find out for yourself". Well guess what, a lot of the times, if the simulation finishes (successfully) with a high-res mesh, it finishes with a low-res mesh as well! And if it fails for low-res mesh, it fails for high-res mesh as well! (note: I'm not saying I don't know high-res mesh is good from the numerical POV. I know that. What I'm wondering is, when is low-res the cause of convergence failure and when it isn't and what are the scales of hi vs lo-res, 100 per micron? 1000? 10k? 100k?)
  • I read one of the TCAD "guidelines" that said "Always use the keyword Extrapolate in Math section (unless case A, B, C etc)". Well guess what, that's a garbage advice, from the official source no less. I spent a month dealing with one convergence failure after another only to find out removing the 'Extrapolate' flag fixed the failures for a huge number of configurations. And no, my configs did not fall into the A, B, C cases which are the documented exceptions.
  • In reverse bias, the pre-breakdown current is really low. Is that the cause of convergence failure? as opposed to a bad mesh? How do I find that out I'm completely wasting my time mis-diagnosing the convergence problems?
  • Don't even get me started on time-step adjustment. Nothing could be more ad-hoc than that.

The whole thing appears to be based on trial-and-error, and once things get working, we are none-the-wiser about why things failed, and how to avoid failure in future simulations.
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« Last Edit: Mar 17th, 2017, 9:18am by physixer »  
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