Si/SiO 2 substrates bearing single stranded homooligonucleotidic sequences are implemented, in order to use as DNA biosensors based on field effect measurement. The oligonucleotides are immobilized on the substrates by two different methods, with either an aminosilane or a glycidoxysilane. We present investigations and characterizations of both methods of immobilization, using radiolabelling and electrochemical impedance measurements on electrolyte/dielectric/semiconductor heterostructures. In our conditions of reaction, the protocol using glycidoxysilane allows a denser and more robust immobilization of oligonucleotides than with the aminosilane.