We utilize a combination of DC-IV measurements as well as two very sensitive electrically detected magnetic resonance measurements, spin-dependent recombination and spin dependent tunneling, to identify the atomic-scale defects involved in the negative bias temperature instability (NBTI) in 2.3nm plasma-nitrided SiO2-based pMOSFETs. Our measurements indicate that the NBTI-induced defect in the plasma-nitrided devices participates in both spin-dependent recombination and spin-dependent tunneling. The high sensitivity of our spin-dependent tunneling measurements allow for an identification of the physical and chemical nature of this defect through observations of 29Si hyperfine interactions. We identify these defects as silicon dangling bond defects in which the central silicon is back bonded to nitrogen atoms. We assign these NBTI-induced defects as KN centers because of their similarity to K centers observed in silicon nitride. The defect identification in plasma-nitrided devices helps to explain: (1) why NBTI is exacerbated in nitrided devices and (2) conflicting reports of NBTI induced interface states and/or bulk traps.