The surface recombination at the Si/amorphous hydrogenated silicon nitride (a-SiNx:H) heterojunction is investigated by transient photoconductance measurements as a function of the applied potential. A minimum of the photoconductance decay time is observed at the flatband potential. The fast decay at this potential, compared to the slow decays under accumulation and depletion regimes, indicates that surface passivation of silicon by an a-SiNx:H layer is mainly due to field passivation caused by the fixed charge in the a-SiNx:H layer. The present measurements suggest that the fixed charge is not stable under the application of a potential, at least in the electrochemical cell configuration studied here. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3453568] All rights reserved.