Double-layer coupling dielectrics for AC-coupled Si microstrip detectors have been electrically characterized in order to determine their performance in a radiation-harsh environment, with a focus on the dielectric breakdown. Two different dielectric technologies have been investigated: SIO2/TEOS and SiO2/Si3N4. Dielectrics have been tested by using a negative gate voltage ramp of 0.2 MV/(cm.s). The metal/insulator/Si I-V characteristics show different behaviours depending on the technology. The extrapolated values of the breakdown field for unirradiated devices are significantly higher for SiO2/Si3N4 dielectrics, but the data dispersion is lower for SiO2/TEOS devices. No significant variation of the breakdown field has been measured after a 10 Mrad (Si) gamma irradiation for SiO2/Si3N4 dielectrics. Finally, the SiO2/Si3N4 DC conduction is enhanced if a positive gate voltage ramp is applied with respect to the negative one, due to the asymmetric conduction of the double-layer dielectric
