We measured the response of reverse-biased a-Si:H diodes, in the p-i-n geometry, to charged particles as a function of ionization, detector bias (V) and angle of incidence, in the ionization range of 10–300 keV/µm. In the two cases where the amount of signal was weak, namely at low biases or at low ionizations, the signal was extracted from the noise using a coincidence technique. The response was found to be roughly proportional to , at all values of dE/dx, in the bias range of 7 V to 50 V. The dependence of the charge collection efficiency on the ionization density indicates a strong plasma effect in the detector. This collection efficiency increases steeply when the ionization is decreased but no plateau has yet been reached, indicating that the plasma effect is still important at 10 keV/µm.