Experimental results prove that the dielectric breakdown and the current conduction of the oxide/nitride/oxide (ONO) structures indicate symmetric characteristics related to the gate bias polarity. This is due to the thick bottom and top oxides. The conduction current in the ONO structures is remarkably reduced as the bottom or top oxide thickness becomes thicker than 3 nm. The time-dependent dielectric breakdown increases with increasing bottom-oxide thickness under negative gate bias and with increasing top-oxide thickness under positive gate bias. These two phenomena are due to the change of the oxide barrier for a hole. 3 nm is the transition thickness for the current conduction and breakdown mechanisms. The dielectric breakdown and the current conduction of the ONO structures show symmetric characteristics related to the gate bias polarity due to the bottom and top oxides