The highly sensitive surface of porous silicon (PSi) and the possibility to measure changes in its electrical properties can be used for electrical biosensor applications. The sensing scenario can be extended from the well-characterized organic solvents and pH sensors to detection of biological processes involving the presence of charged molecules. We have investigated the change in electrical properties (capacitance and conductance) of macroporous silicon layers upon exposure to organic solvents and water. Evaluation of our sensor to detect a biological event was done by addressing DNA hybridization. As opposed to the previous work published in this field, in our devices the electrical contact is made on the crystalline silicon (c-Si) substrate. This allows a complete exposure of the surface to the sensing species and reduces the generation of ionic currents through the porous silicon matrix. We will report results of a complete characterization of our device including response speed, selectivity and sensitivity.