Recent results of global thermal field modelling using the software programs CrysVUN++ and STHAMAS are presented for 3- and 4-inch VCz SI GaAs growth assemblies. For the first time global simulations including the gas and melt convection in a VCz arrangement are shown. In contrast to conventional LEC the impact Of gas convection on the temperature distribution in the growing crystal is not dominant in the VCz case. Contrary to that melt convection cannot be neglected and needs to be controlled by crucible and crystal rotation rates. Simulations were used to optimise the temperature fields in the VCz arrangements and thus, to reduce the von Mises stress within the growing crystal. Doing this, the EPD could be reduced by approximately one order of magnitude compared to conventional LEC Minimum EPD's were found to be in the range of some 10(3) cm(-2) for 4 " crystals. The observed nearly flat (slightly convex) interfaces are in agreement with the simulations. The achieved good radial uniformity of the carbon incorporation and electrical parameters is due to the improved thermal boundary conditions.