Off-state gate oxide breakdown (BD) under non-uniform electric field is performed in 28nm FDSOI MOSFET devices. We investigated the impact of SILC, channel current and lateral electric field in dielectric BD mechanism related to RF operations and ultra-short channel devices. Oxide BD is characterized under DC stress with different gate-length LG as a function of drain voltage VDS and temperature. We check that the leakage current is the better monitor for TDDB dependence precursor to Hard-breakdown (HBD) under Off-mode stress by using the proper modeling and discussing the different possible origin of the higher form factor  value for PFET under Off mode stressing due to high impact ionization and non-conducting hot-carrier.