We made a characterisation of GaAs detectors with an epitaxial p-type layer deposited on the 200μm semi-insulating substrate. The charge collection efficiency for 60-keV photons and 5.49 MeV alpha particle depends on the doping level of the p-layer. When completely depleted (reverse bias > 200-300 V), the collected charge can be greater than 100%, implying the presence of some charge gain mechanism. At the same reverse bias and doping concentration, the collected charge depends also on the size of the contact pad. Moreover, the lower the p-doping, the lower the current density. The present findings confirm our previous work, obtaining gains up to 4:1.
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