The response-speed of Si-based metal-semiconductor-metal (MSM) photodetectors was improved by depositing a composition-graded intrinsic hydrogenated amorphous silicon-germanium (i-a-Si1-xGex:H) layer on crystalline silicon (c-Si). In contrast to the non-composition-graded one (using intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer), the full width at half maximum (FWHM) and fall time of the photodetector transient response were improved from 145.2, 404.6 to 107.6, 223.4 ps respectively. The experimental results showed that the device responsivity and quantum efficiency were increased from 0.329 (A/W) and 0.492 to 0.414 and 0.619 respectively by the employed composition-graded technique. We propose that this enhancement is due to a smoother barrier that is formed at the c-Si and i-a-Si1-xGex:H interface. A lower deposition temperature of i-a-Si1-xGex:H layer could be used to further reduce the fall time of the device transient response from 315.6 (250 degreesC) to 97.6 (180 degreesC) ps. To improve the contact properties between Cr electrode and i-a-Si1-xGex:H layer, an annealing technique in hydrogen ambient was employed. The device knee voltage, which is the applied voltage at which the device current start to enter the saturation region in its current (log-scale) versus applied voltage characteristics, could be reduced to around 3.5 V after annealing. (C) 2002 Elsevier Science Ltd. All rights reserved.