The quasibreakdown (QB) of ultrathin gate oxide under substrate hot-hole and -electron injection was investigated. It has been found that hot-carrier injection greatly degrades the immunity to the QB and the hot hole is more effective than the hot electron in the degradation. The phenomenon was explained by an interface damage-controlled mechanism and verified again by monitoring the interface state densities at the onset point of QB. It also has been found that QB occurs when the interface state density reaches a critical value, regardless of the stress current density and stressing carrier type. (C) 1999 American Institute of Physics. [S0021-8979(99)00523-X].