Changes in the characteristics of a µc-Si:H-emitter Si-HBT induced by forward current stress imposed on the emitter-base junction are studied. The principal changes appear in the base current which consists of the diffusion and the band-gap-state-assisted tunneling currents. The diffusion current at first increases and later decreases as stressing proceeds; the overall result due to long-term stressing is reduction. Consequently, the maximum hFE, governed by the diffusion current component, is enhanced as a result of stressing. The tunneling current increases monotonically with increasing stress time, resulting in degradation of hFE at low output currents. Changes in both diffusion and tunneling currents can be explained by the simultaneous occurrence in µc-Si:H of increases in the active dopant and the defect densities.