The change in the threshold voltage 
, the signature of NBTI, originates from
trapped charges which are immobile and therefore cannot carry any drain current.
Hence, the central question arises whether NBTI must be ascribed to charges trapped
at the 
interface (interface traps), in the dielectric (oxide traps), or even a
combination thereof.
Interface traps stem from the lattice mismatch caused by the abrupt transition from
crystalline bulk silicon to amorphous 
. Even the amorphous nature of modern
high-quality oxides is not fully capable of compensating the lattice mismatch through
the flexibility of its bonding network. As a result, a certain fraction of 
atoms
cannot establish four bonds to their neighbor atoms and thus leave behind
unsaturated bonds, the so-called dangling bonds [8, 9]. The corresponding orbitals
can carry up to two electrons and feature two trap levels found to lie within the
substrate bandgap. Three different types of interface defects are observed
experimentally: 
centers have been found at 
interfaces, while 
and 
centers are present at the technologically more relevant 
interfaces [8, 10, 11]. The creation or annealing of charged interface states induces
a non-negligible threshold voltage shift 
, which is considered as an
undesired degradation by engineers. However, these states can be eliminated by
exposure to a hydrogen ambient, where the interfacial dangling bonds are
passivated and their corresponding energy levels are shifted out of the substrate
bandgap [12, 10].
There exists a series of traps, such as cycling positive charges [13], anti-neutralization
positive charges [13], border traps [14], switching traps [15, 16], oxide traps, 
centers [17], and 
centers [18]. However, no precise distinction has been
made between them. Experimentally, these traps are characterized by either
their trapping time constants or their defect structures. Furthermore, their
properties have been found to strongly depend on the local environment
of their host material, such as 
centers in amorphous 
[19, 20].
Nevertheless, all these types of oxide traps have been linked to the NBTI
phenomenon [21, 22] since they are capable of exchanging charge carriers with the
substrate. Electron or hole injection is assumed to proceed by some kind of
elastic [23] or inelastic trapping mechanism [24] into spatially and/or energetically
distributed traps. Ongoing research is now dealing with the exact physical
description of this process, including all dependences on the oxide field and the
temperature.