Discussion Forum

Hi! I'm trying to learn Comsol, but it seems have run into a wall. I've looked thoroughly through the semiconductor_diode model as well as the MOSFET model (in the showroom). However, as I implement these models myself, following the instructions point by point, the model diverges.

In general, I've noticed that I can get good convergence in a single Poisson simulation of a piece of semiconductor, with a given doping profile. As soon as I add transport of dilute species coupled with Poisson, it diverges.

In order to find my errors, I've created a very simple semiconductor model. It's basically a 1umX10um piece of silicon with a homogeneous doping concentration (but engineered such that it can be changed later). I provide an analytic initial value to an electrostatics module, which works fine and it solves the poisson equation. I then feed this solution to an electrostatic module coupled with two 'transport of dilute species' module, one for electrons and one for holes. As soon as this runs however, the model diverges.

I've attached the model (Comsol v. 4.1) and would be very grateful if someone with experience could point me in the right direction. I must be missing something fairly simple. If 'step 1' is solved, you see the potential for the first poisson solver (constant due to constant doping density). Step 2 then tries to solve poisson coupled with diffusion using the first solution as initial value, but diverges.

The essential variables are:
n0,p0,phi_init: analytic initial values fed to the first poisson solver.
phi0: potential solved by first poisson
phi,cn,cp: potential, electron and hole concentrations for second solver step.

I've been staring at this for a few days now and if I don't find the problem soon, I will have to give up using Comsol.

Thanks!