Copper Heat Sink with GaAs atop

Hello all.

I am trying to model a problem attached as a picture, it consists of a copper heat sink, and a GaAs block atop it. A voltage of 10V is applied as a Terminal to the GaAs while ground exists at the base of the heat sink. However, when modelling this problem I encounter an error, the relative error is greater than relative tolerance error.

Wondering if you have any suggestions how to fix this problem. I can upload my mph file if needed.

Thank you.