Negative bias temperature instabilities are studied on metal-oxide-semiconductor structures, with hydrogen-passivated and depassivated (H-free) (111) and (100)Si/SiO2 interfaces. By combining electrical and electron-spin resonance measurements, it is found that P-b/P-b0 centers are generated on passivated surfaces, while their density is reduced on depassivated surfaces. These results suggest the liberation of atomic hydrogen species in the gate stack followed by their transport towards the Si/SiO2 interface and reaction with the P-b and PbH centers. It is shown that these species are likely not released from the Si substrate. Alternatively, the authors propose that they may result from the cracking of hydrogen- and water-related species at or close to the metal/SiO2 interface when the device is subjected to negative bias temperature stress. (c) 2007 American Institute of Physics.