A thin (30 nm) hydrogenated-amorphous-silicon (a-Si:H) layer has been introduced to dramatically suppress the dark current of SiGe/Si metal-semiconductor-metal photodetectors (MSM-PDs) and enhance their photocurrent, around 850 nm wavelength. The effects of a-Si:H as a passivation and a Schottky-barrier-height (SBH) enhancement layer on reducing the dark current of SiGe/Si MSM-PDs have been studied in detail. It was found that the a-Si:H as a passivation layer could effectively suppress the dark current better than the a-Si:H as a SBH enhancement layer. Compared to uncapped samples, the samples with a-Si:H passivation layer could enhance the photo-to-dark-current ratio and responsivity by a magnitude of 1.39x10(6) and 21, respectively. The related mechanisms have been proposed in this paper. (c) 2007 The Electrochemical Society.