Various amorphous silicon alloy films (i.e. i-a-Si:H, i-a-Si0.65Ge0.35:H, and i-a-Si0.56C0.44:H) were deposited on crystalline silicon (c-Si) wafers to form amorphous-crystalline heterojunctions which could be used to enhance the performance of planar Si-based metal-semiconductor-metal photodetectors (MSM-PD's). The photocurrents (dark currents) of the fabricated devices were slightly (significantly) lower than that of the MSM-PD without amorphous layer, and the FWHM (full-width at half-maximum) of the temporal response for MSM-PD could be reduced by employing an amorphous film. Experimentally, at a bias voltage of 20 V and for a 830 nm incident semiconductor laser power of 10 mu W, the Si-based MSM-PD with a 50 x 50 mu m(2) active area and an i-a-Si0.65Ge0.35:H overlayer had a responsivity of 0.35 A W-1, a low dark current density of 0.4 pA mu m(-2), a narrower FWHM of 53 ps, and a shorter transient tail of 399 ps for its temporal response. The spectral response of a Si-based MSM-PD with an additional amorphous overlayer had a peak around 700 nm, and covered the range 500-900 nm including 830 nm. (C) 1997 Elsevier Science Ltd.