The shift of the threshold voltage, V-th, of p-channel metal-oxide-semiconductor field-effect transistors with HfSiON gate stacks, subjected to negative bias temperature stress, is investigated. It is found that Vth shift increases with time, like a power law, with an exponent that decreases with the amount of Hf in the HfSiON layer. Within the reaction-diffusion model for negative bias temperature instabilities, this finding suggests that the transport of hydrogen species in the gate stack is slower (i.e., more dispersive) in Hf-rich silicate layers. This result is also consistent with the observed increase in activation energy related to the ho ping of the species in the gate stack with the p Hf content, extracted from the temperature acceleration of V-th shifts. It is also found that negative bias temperature instabilities are reduced in layers with Hf content of about 50 at. %. This ''optimum" is tentatively attributed to the interplay between the slower transport of the hydrogen species and the larger density of interface and bulk defects in the gate stack when the Hf content increases. (c) 2005 American Institute of Physics.