Unoxidized and oxidized porous silicon (PS) based metal/PS/c-Si photodetector structures have been prepared by anodization of 10 and 20 000 Omega cm p-type silicon (c-Si) substrates. A quantum efficiency up to 0.7-0.8 in the wavelength range 0.4-0.7 mum, a detectivity up to 6 x 10(11) cm Hz(1/2)/W and a response time less than 2 ns were obtained. It is shown that the noise characteristics of the best device structures are determined by a generation current. We found that in oxidized device structures and structures made from highly resistive substrates a highly conductive inversion (n-type) layer is formed in the c-Si substrate adjacent to the PS/c-Si heterojunction. The presence of this inversion layer leads to an increase of the active device area, capacitance, noise current and response time.