  The radiation damage in 200 um thick Schottky diodes made on semi-insulating
(SI) undoped GaAs Liquid Encapsulated Czochralski (LEC) bulk material with
resistivities between 0.4 and 8.9*10E7 Ohm*cm were studied using
alpha-spectroscopy, signal response to minimum ionising particles (MIP), I-V
and CV-measurements. The results have been analysed to investigate the
influence of the substrate resistivity on the detector performance after
neutron and proton irradiation. The leakage current density, signal response to
alpha-particles and MIPs show a strong dependence on the resistivity before and
after irradiation. An observed decrease of the electron mean free drift length
before and after irradiation with increasing substrate resistivity can be
explained by a model involving the different ionisation ratios of defects,
which are introduced by the irradiation. Comparison of the radiation damage due
to neutrons and protons gives a hardness factor of 7+-0.9 for 24 GeV/c protons.
The best detectors show a response to MIPs of 5250 e- at 200 V reverse bias
after a irradiation level of 2*10E14 p/cm^2.
