According to the current understanding BTI is either due to the creation of
interface states at the Si-
interface, generally known as 
-centers, and/or
to the trapping of positive charge inside the 
, possibly at so-called
-centers. In order to link the observed degradation to these two types of
defects as possible underlying physical origin of the BTI phenomenon, the
definition
 | (1.1) |
with 
as oxide charge, 
as the charge stored in the
interface traps, and 
as the interface trap density and 
as the occupancy
function at the interface is used. Unfortunately, these charges, whose sum is
directly related to the change in 
, cannot be measured directly. Also,
the relative contribution of 
and 
does not necessarily have to
be constant over 
. Nevertheless, till now the defects causing 
and 
are assumed to be the most likely explanation to why BTI
happens.
So far, a large number of recent publications have tried to explain NBTI, and in recent years also PBTI. Numerous methods have been devised to classify the contributing defects, their number, spatial positions and energies, and time response. Also, the possible mechanisms of defect creation were thoroughly studied, but up to now, no consensus has been achieved and the debate on the underlying physics and its consequences continues.