In the present work, electrochemical impedance measurements in combination with conductance measurements in the microwave frequency range were applied to semiconductor/insulator heterojunctions in an electrochemical cell configuration. It is shown that the combination of both techniques yields a more reliable and detailed interpretation of the experimental results. The Si/a-SiNx:H interface was characterized by three different methods. Quasi-stationary photoconductance measurements in combination with photocurrent measurements were performed as function of the external cell polarization. Potential modulated conductance and impedance measurements were used to characterize the electrical properties of the Si/a-SiNx:H interface. The reliability of the Mott-Schottky procedure was successfully tested for two different electrochemical cells. Transient photoconductance measurements as a function of the applied potential were used to determine the surface recombination kinetics at the active interface. The experimental results led to the conclusion that the fixed charge in a-SiNx:H is responsible for the electrical passivation of Si wafer. Furthermore, it was observed that the electronic configuration of this interface presents certain instabilities when driven in aqueous media. (C) 2013 Elsevier Ltd. All rights reserved.