For relaxation, based on the assumption of full recovery after each stress/relaxation cycle,
is chosen with
being the pulse period, and implying
. Therefore
is independent of
, whereas
depends on the
used. Note that due to record length constraints of
the DSO, not the entire relaxation characteristic up to
is recorded,
but only the initial relaxation up to around three to four times
.
The point
nevertheless is available in the pre-trigger data of the
DSO.
Data extraction turns out to be extremely sensitive to the choice of ,
indicating that the settling time of
plays a crucial role in OTF experiments.
To demonstrate this fact, Fig. 6.18 shows relaxation after
for
different values of
. If the criterion is too conservative, i.e.
is chosen
small, thereby cutting off the initial relaxation phase, the shape of the
relaxation characteristics is significantly altered. On the other hand, too large
values of
, i.e. too liberal limits for gate voltage settling, may produce
spurious relaxation transients. In other words, with different
values
more or less points are considered for the relaxation curves leading to
different initial slopes. The ‘real’ initial data, as seen in Fig. 6.18, then
becomes artificially dispersed when too many data points are cut off (small
).
Assuming the relaxation follows as indicated by the red curve in
Fig. 6.18, and setting the starting point of the extracted relaxation to later times
(through smaller
) gives a dependence of
, which produces
the artificial plateaus seen with the blue curves in the figure. This may lead to
the wrong conclusion that the time constants are smaller than they actually
are. Possibly the saturation towards smaller relaxation times found in
[42, 102, 107] could be explained that way, i.e. the plateaus observed are not a
feature of NBTI relaxation but an artifact due to finite settling times and
synchronization inaccuracies, in turn invalidating the assertion that a
measurement delay in the micro-second regime is sufficient to correctly capture
the relaxation characteristics of NBTI. Besides, though an
of 0.025
seems to be quite large, the resulting
only lies within
of the
settled
. Therefore these values well account for the relaxation
region.