We have compared the thermal damage in ultrathin gate SiO2 layers of 5.6 and 3 nm thickness after intrinsic dielectric breakdown due to constant voltage Fowler–Nordheim stress. The power dissipated through the metal–oxide–semiconductor capacitor during the breakdown transient, measured with high time resolution, strongly decreases with oxide thickness. This is reflected in a noticeable reduction of the thermal damage found in the structure after breakdown. The effect can be explained as the consequence of the lower amount of defects present in the oxide at the breakdown instant and of the occurrence of a softer breakdown in the initial spot. The present data allow us to estimate the power threshold at the boundary between soft and hard breakdown, and they are compared to numerical simulations of heat flow. © 2001 American Institute of Physics.