A simplified approach is used to determine the relative importance of slow (>3 s) and fast (<3 s) charged trap contributions to threshold voltage shifts (Δ Vth) induced by negative bias temperature instability in HfSiON gate dielectric p -channel field effect transistors. For the devices under study the relative importance of the two components is approximately the same. Total trap density induced threshold voltage shifts from measurements at 368, 398, and 428 K can be fitted to a simplified law of the form Δ Vth=A e-EA/kTtαwith EA=0.085±0.012 eV and α=0.186±0.003. The importance of ignoring fast trap effects in overestimating reliability lifetimes is discussed. © 2009 American Institute of Physics.