The conduction mechanism of quasi-breakdown (QB) for ultra-thin gate oxide has been studied in dual-gate CMOSFET with a 3.7 nm thick gate oxide. Systematic carrier separation experiments were conducted to investigate the evolutions of gate, source/drain, and substrate currents before and after gate oxide QB. Our experimental results clearly show that QB is due to the formation of a local physically-damaged-region (LPDR) at Si/SiO2 interface. At this region, the effective oxide thickness is reduced to the direct tunneling (DT) regime. The observed high gate leakage current is due to DT electron or hole currents through the region where the LPDR is generated. Under substrate injection stress condition, there is several orders of magnitude increase of Isub(Is/d) at the onset point of QB for n(p) - MOSFET, which mainly corresponds to valence electrons DT from the substrate to the gate. Consequently, cold holes are left in the substrate and measured as substrate current. Under gate injection stress condition, there is sudden drop and even change of sign of Isub(Is/d) at the onset point of QB for n(p)-MOSFET, which corresponds to the disappearance of impact ionization and the appearance of hole DT current from the substrate to the gate. In the LPDR region, the damaged structure may have two or multi metastable states corresponding to different effective oxide thickness. The thermal transition between two or multi metastable states leads to random telegraph switching noise (RTSN) fluctuation between two or multi levels.