The experiment result shows that under unipolar voltage stress, electrons tunneling into ultra-thin gate oxide during on-time will recombine with the trapped holes, and neutral electron traps would be generated. These electron traps would assist electrons tunneling into gate oxide. It is proposed that the characteristic time τ for the charge to be trapped in SiO2 is longer than the on-time ton of unipolar stress. During the on-time few charges are trapped and during the off-time some trapped charges will be detrapped, thus very few neutral electron traps are generated. With the stress time increasing, the density of neutral electron traps would reach a critical value, and then the gate oxide would break down abruptly. The increase in unipolar breakdown time tBD is attributed to fewer charge trapping during the on-time and charge detrapping during the off-time.