Thin-film ionizing radiation detectors prepared from amorphous semiconductors are receiving a growing interest. To reach ultimate sensitivity, single particle detection is needed. Amorphous silicon detectors operated in this mode require a high polarization to overcome the limitations due to the low electron and hole effective mobilities and to charge trapping. In this review, we summarize results obtained on charged particle detection (electrons, protons and alphas) with special emphasis on the electron-hole pair generation and collection mechanisms. We report a study on the role of p-doped layers in obtaining high breakdown voltages (more than 7 × 105V/cm). Physical mechanisms underlying leakage current injection and generation are discussed.