The frequency (ω) response of the admittance (Y) of Czochralski (Cz) and Bridgman (Bg) grown semi-insulating materials have been investigated in the range 300–500 K. For both materials, this conductivity remains constant up to a frequency ω at which it becomes proportional to ω1. A minimum is observed in the Y(ω) characteristics between these two regimes in Cz materials but not in Bridgman ones. It is suggested that the existence of this minimum is related to percolation induced by the presence of space-charge regions located around As precipitates and charged dislocations, which are present in Cz but not in Bg materials. The percolation invalidates the homogeneous conduction assumption made in analyzing the electrical properties of semi-insulating Cz materials. © 1997 American Institute of Physics.