This study investigates how different metal gate fabrication methods induce variations in the defects which result from nitrogen diffusing into the hafnium oxide layer in metal-oxide-semiconductor field effect transistors (MOSFETs). By using the different fabrication methods of pre-TaN, post-TaN and post-TiN annealing, the work-function difference between the gate material and the semiconductor can be adjusted, leading to apparent differences in threshold voltage (V-th). In addition, the results of slow and fast I-V NBTI measurements show that the amount of the bulk trapping in the post-TiN device is the highest, followed by the post-TaN device and then the pre-TaN device. In addition, a nitrogen interstitial defect phenomenon, resulting in a temporary shift of threshold voltage (V-th) which is highest in the post-TiN the lowest in the pre-TaN device, is determined by double sweep fast I-V measurements. (C) 2016 Elsevier B.V. All rights reserved.