The influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by rf-PECVD, is studied by constant photocurrent method, CPM, and by slow positron beam spectroscopy, respectively. Deep defect density, NDD, remains approximately constant at 10-16 cm-3, typical of device quality material, for PD in the range 7 - 20 mW.cm-3 calculated from CPM. NDD increases roughly one order of magnitude for both low and for high power densities. Positron annihilation spectroscopy reveals the presence of two kinds of vacancy type defects in the films: large vacancy clusters or voids for PD ∼ 7 mW.cm-3 and small vacancy type defects as PD increases to ∼30 mW.cm-3. The relative abundance of the structural defects is shown to be controlled by the power density used during the deposition without affect the electronic deep defect density, significantly.