In the middle of the last century, McWhorter [182] dealt with the spectrum of
-noise observed at germanium-oxide interfaces. This kind of noise is attributed
to fluctuations in the trap occupancy
due to charge carriers tunneling forth
and back between the bulk and the defects. McWhorter described these
fluctuations using a simple SRH-based model, which can be considered as a
prototype for other charge trapping models. His model extends the conventional
SRH theory by the effect of charge carrier tunneling, which is accounted for
by the factor
. Thus, the simplified time constants read as
In the following, the McWhorter model will be evaluated against the findings of the TDDS experiments (see Section 1.3.4).
The term in
and
accounts for the trap depth
dependence of tunneling and leads to an upwards shift of the entire
and
curves with an increasing trap depth
. Due to the wide distribution of
,
the McWhorter model allows a wide range of capture and emission times in thick
oxides. In modern device technologies, however, the time constant of the
devices with an oxide thickness of
would be limited to
after
the model. As such, this model cannot explain time constants larger than
for devices with an oxide thickness of
. This is in contrast to the
experimental results (cf. Fig. 1.2), in which
extends well into the
kilosecond regime. In conclusion, this model cannot be reconciled with the
findings of the TDDS and is thus inadequate to describe the traps involved in
NBTI.