ABSTRACTA series of pure hydrogenated amorphous silicon (a-Si:H) samples as well as carbon (aSi:C:H) and sulfur alloys (a-Si:S:H) were investigated by means of the photoconductive time-of-flight technique. Drift moblility (µd) measurements reveal a fast decrease of the electron µd upon C or S addition, while the hole µd does not change significantly. Contrary to the electron transients, hole transient currents do not show the typical space-charge-limited (SCL) features, even for high light intensities where these features are normally seen. SCL features are observed to disappear for electron transients as well in the a-Si:H alloys with increasing alloy content. Above results are all explained starting from the a-Si:H density of states (DOS) model where the valence band (VB) tail is much broader than the conduction band (CB) tail. Introducing additional disorder by alloying broadens both the VB and the CB tails while the relative increase of the CB tail is much greater than that of the VB tail.