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Soft breakdown in ultrathin gate oxide has been studied using constant voltage stressing. The behavior of current increments resulting from a number of soft breakdown events has been characterized by statistical distribution. It is shown that the distribution of the current increment follows Weibull distribution rather than log normal distribution. The newly established Weibull slope is shown to be independent of the stressed voltage in the range investigated between 4.5 and 5.1 V. The temperature effect study shows that the Weibull slope reduces with increasing testing temperature. Furthermore, a strong dependence of the Weibull slope on the oxide thickness has been found. These observations can be explained well by geometrical configurations of the percolation path.