The electronic transport properties and structural morphology of fast-pulse excimer-laser- crystallized hydrogenated amorphous silicon (a-Si:H) thin films have been measured. The room-temperature dark dc conductivities and Hall mobilities increase by several orders of magnitude at well-defined laser energy density thresholds which decrease as the impurity concentration in the films increases. The structural morphology of the films suggests an impurity-induced reduction of the a-Si:H melt temperature as the origin of this behavior.