In this paper, a detailed study of charge relaxation in high-k metal-gate nFETs is reported. We show that independent fast and slow relaxation occur in parallel. While the slow charge relaxation follows the typical universal dependence on t relax/t stress, the fast transient relaxation does not show stress time dependence for the stress time range investigated in this work (t s>∼200us). Study of the dependence on high-k thickness and electric field also reveals very different characteristics between the fast and slow BTI and suggests the fast BTI is likely caused by a specific group of defects with very short capture time near Fermi level in the high-k layer. © 2012 IEEE.