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
degradation2 .
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].