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In this paper, we report, for the first time, the statistical distribution of thin-gate-oxide breakdown characterized by using conductive atomic force microscopy (C-AFM) in conjuncdion with the semiconductor parameter analyzer Agilent 4156C. Nanoscaled constant-voltage stress (CVS) and constant-current stress (CCS) were applied to the samples, and the time-to-breakdown Tbd Weibull plots were obtained as a function of stress condition as well as oxide thickness. A different trend in TBD Weibull distribution dependence on the oxide thickness was found for the case of nanoscaled CVS and CCS. By examining the pre- and post-breakdown I-V characteristics as well as their curve fittings, we found that different degradation mechanisms are involved in the oxides subjected to CVS and CCS. For oxides subjected to nanoscaled CVS, the degradation follows the percolation model, whereas for those subjected to nanoscaled CCS, the degradation obeys the trap-assisted tunneling model. The Weibull slope beta value obtained in this paper is found to be consistent with those in the conventional oxide-breakdown tests.