Porous silicon (PS) based metal/PS/p-Si structures with PS layer of different thickness were prepared on moderate- and high-resistivity substrates. Measurements of current-voltage (I-V) characteristics and impedance at various temperatures were used for the investigation of the electrical properties of these structures. Electrical properties of the structures with relatively thin (I Am) PS layer significantly differ from those of thick structures. The exponential forward bias I-V dependencies for thin structures spread over several orders of magnitude with a low value of quality factor (close to 2) and have activation temperature dependencies with an activation energy equal to half the c-Si band gap. The reverse current has a square root dependence on the reverse bias voltage and the activation energy is equal to half the c-Si band gap. Therefore, it was concluded that the reverse and forward currents in thin PS-based device structures were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was shown that a large area spreading current exists in structures made on highly resistive substrates, which appears to be due to a highly conductive inverse (n-type) layer formed in the p-Si substrate at the PS/p-Si heterojunction. The spreading effect leads to high reverse currents and high capacitance of the device structures made on highly resistive substrates. (C) 2001 American Institute of Physics.