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Transmission electron microscopy (TEM) has been successfully applied to study the microstructural defects responsible for the breakdown in ultrathin SiOxNy and Si3N4, and HfO2 gate dielectrics. Contradicting the general belief that gate oxide defects are confined within the gate dielectrics, the TEM analysis reveals that the physical defects associated with the gate dielectric breakdown involve both the gate electrodes i.e., poly-Si gate and Si substrate. High resolution TEM and chemical/elemental analysis in TEM show that regrowth of Si epitaxy, gate dielectric thinning, silicide migration, poly-Si gate melt-down and recrystallization, Si substrate point defects and metallization/contact burnt-out are common gate dielectric breakdown induced failure defects, and their presence depends strongly on the growth of the hardness of the breakdown. In this paper, we present a detailed TEM analysis on the main microstructural defects responsible for the breakdown of ultrathin gate dielectrics.