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Figure 6.1 shows the measured figure of merit,
. It represents the
accumulated electron density in the silicon bulk close to the
interface at flat-band conditions.
is extracted from the flat-band
voltage which is obtained by capacitance-voltage measurements (Section 4.3).
At that time the effect was remarkable, as a major concern was the migration of
mobile ions due to the electric field. For negative gate bias stress this
would suggest positive ions from the oxide to be attracted by the negative gate
electrode leading to a ``more negative'' dielectric and thus to a decrease of
for p-channel MOSFETs. This effect can be seen in
Figure 6.1 for gate voltages down to
V. At more negative stress
voltages another degradation mechanism prevails, leading to positive charging
of the dielectric and/or the
interface and therefore to an increase of
.
Miura and Matukura proposed an electrochemical reaction under the influence of
the strong electric field at the
interface. This reaction leads to
positively charged oxygen vacancies in the
film. As this mechanism
proceeds at higher electric fields it dominates the ion migration process which
saturates at large biases.