Threshold voltage V-th instability is one major issue for future metal-oxide-semiconductor field effect transistors (MOSFETs) with hafnium (Hf) based gate dielectrics. Previous attention was focused on n-channel MOSFETs (nMOSFETs) and the implicit assumption is that it is not important for p-channel MOSFETs (pMOSFETs). This work shows that the V-th instability of pMOSEFTs can be higher than that of nMOSFETs for a sub-2 nm nitrided Hf layer. Unlike nMOSFETs, the V-th instability of pMOSFETs is insensitive to measurement time, does not saturate as stress voltage increases, and is not controlled by carrier fluency. Using Hf silicates is less effective in suppressing it. Some speculations are given on the defect and physical processes responsible for the instability. (c) 2007 American Institute of Physics.