An account is given for the conditions under which the collection efficiency in hydrogen ated amorphous silicon pin-diodes increases to values larger than 100 %. By specific bias illumination through the p-side bias generated photocarriers are collected under certain probe beam conditions of the collection efficiency measurement, leading to apparent large collection efficiencies. By numerical modelling we investigated the influence of the diode thickness, bias photon flux and probe absorption coefficient as well as applied voltage for possible sensor applications which may utilise this optical amplifying principle. The alternative with bias light through the n-side and probe light through the p-side is also explored. Collection efficiency values determined by the photogating of bias generated holes become only slightly larger than 100 % in contrast to the electron case where values in excess of 3000 % are presented.