The simulation area is bounded assuming Neumann conditions for the electrical and thermal boundary conditions, setting = 0 and = 0. For HEMTs, which generically are multifinger devices with up to eight gate fingers in the third dimension, this is a good assumption for the electrical properties. However, for the solution of the heat flow equation this approximation introduces serious errors since the gate to gate finger interaction cannot be neglected in multifinger devices, as was shown e.g. in . A sound treatment of the thermal boundary conditions is carried out in Chapter 6.