The conduction mechanism of quasibreakdomn (QB) mode for 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 quasibreakdown (QB), Our experimental results clearly show that QB is due to the formation of a local physically-damaged-region (LPDR) at Si/SiO2 interface , tit 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. Twelve V-g, I-sub, I-s/d versus time curves and forty eight I-V I curves of carrier separation measurements have been demonstrated. All curl-es can be explained in a unified wag by the LPDR QB model and the proper interpretation of the carrier separation measurements. Particularly, under substrate injection stress condition, there is several orders of magnitude increase of I-sub(I-s/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. These cold holes have no contribution to the oxide breakdown and thus the Lifetime of oxide after QB is very long. Under gate injection stress condition, there is sudden drop and even change of sign of I-sub(I-s/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.