The effect of Swift Heavy Ion (100 MeV Si7+) irradiation on electronic-transport of Pd/n-GaAs devices has been studied by I-V and C-V techniques. The chemical compositions of the interface have been studied by XPS/EDAX techniques. It is observed that the irradiated devices show a reduction in current and capacitance by few orders of magnitude. The C-V characteristics show a change in conductivity type from n- to p-type after the irradiation. On hydrogenation, the irradiated devices show a capacitance peak in C-V characteristics, which has been ascribed to As vacancies. The XPS studies of these devices, for various etching durations, show that the ratio of As:Ga has reduced after the irradiation, which indicates the formation of irradiation-induced As vacancies. This reduction in As:Ga ratio is also confirmed by EDAX measurement. The observed conductivity type change from n- to p-type (on the irradiation) seems to be due to the change of substitutional sites of dopant silicon atoms from Ga to As sites due to the irradiation-induced As vacancies. (C) 2004 Elsevier B.V. All rights reserved.