The use of metal gates removes gate depletion and allows aggressive downscaling of equivalent oxide thickness. The impact of changing. gate from poly-Si to metal on device reliability has not been fully addressed. This work focuses on how TaN gate affects the positive charge formation in HfO2/sib(2) stacks. Under positive gate bias stresses, the results show that positive charge formation in metal-gated samples is significantly higher than in poly-Si gated samples. However, positive charge formation is similar in these two types of samples, when stressed under negative gate bias. The results are explained by assuming that there are more hydrogenous species at the metal/ dielectric interface and the hydrogen release from the anode dominates the positive charge formation. The behavior of positive charges in the stack is also compared with that in a single SiO2 layer. (c) 2006 American Institute of Physics.