A heat pipe assisted zone melting furnace for the synthesis and single crystal growth of large (up to 55 mm) diameter GaAs is described.

The high thermal conductivity of the heat pipe and the dynamic profile design makes possible the precise control of the background temperature, hence the arsenic vapour pressure. Because of the well defined narrow molen zone, the number of impurities were few and their concentrations minimized.

A multilevel model was developed to calculate the position of the Fermi level in GaAs. All the electrically-acitve impurity concentrations, indicated by spark source mass spectrographic analysis, were taken into account in the computation. The deduced semiconducting or semi-insulating properties are in agreement with the measured characterisitcs of the investigated crystals. It is shown that, in the case of the large gap semiconductors, only deep-lying compensation results in high resistivity, semi-insulating materials.