We have investigated the defect structure of ion implanted amorphous silicon using in situ and ex situ conductivity measurements and electron paramagnetic resonance technique. Electronic transport occurs by hopping at the high density (∼ 1020/cm3 eV) of deep-lying localized states in the band gap. Annealing at 540°C reduces this density by a factor of 5, causing a decrease by ∼ 3 orders of magnitude in the conductivity of the material. In situ measurements demonstrate that this process can be reversed by ion implantation and that annealing of the defects occurs even at room temperature for ∼ 1 h following irradiation. The defect structure of the material is strongly modified by the introduction of impurities at concentrations higher than 1 at.%. The effects of the impurities on the conductivity and on the paramagnetic defect density of the a-Si are explained in terms of direct bonding of the impurity with the defect …