6.3.5 Extracted Prefactors
The prefactor 
of the log-fit for various 
, 
, and 
is displayed in
Fig. 6.13. In agreement with previous experiments, it is observed that low
results in small temperature activation, while 
larger than the
operating voltage of the MOSFET gives a notable activation energy of 
.
Note that this value is in agreement with activation energies extracted at long
stress times [106]. Fitting the data to a power-law 
results in an
exponent 
for short-term stress, roughly a third of the often reported
of the long-term behavior. This is in very good accordance with the
standard 
for NBTI stress and accounts for a strong 
dependence,
excluding elastic hole tunneling.
Figure 6.14 represents the prefactor 
plotted for different 
at different
temperatures. In the devices with 
, all the stress voltages
are above the operating voltage and result in a marked temperature
activation. For 
the transition from no temperature activation
to temperature activation is observed between 
and
for 
. For the thickest oxides used in this study,
, the applied stress fields are too small to lead to a meaningful
degradation .
Therefore no objective statement can be made on temperature activation
concerning the here presented devices with 
.
However, the experiments performed on devices with smaller oxide
thicknesses support thermally activated tunneling mechanism [98] rather than
elastic (and thus temperature-independent) hole tunneling [94].
of the log-fit, extracted
from three different 
for different 
. An activation energy 
of
about 
is obtained for 
and 
, represented
by the black solid line. Degradation for the 
devices was too
noisy due to too low 
. Scale is equal for all plots.
of the log-fit plotted for different 
with
different temperature 
. While 
shows a clear temperature
activation, 
does not due to the low electric stress field. For
the transition of the temperature dependence is visible at
between 
and 
.