According to the scaling theory outlined in Section 2.1, the
gate dielectric thickness must shrink with every new device generation,
reaching values of 2.2nm, 1.9nm, and 1.4nm for 180nm, 150nm, and
100nm gate length devices [23]. However, the quantum-mechanical
tunneling effect comes into play if the energy barrier between gate and
semiconductor becomes to small. One remedy against this effect is to use
dielectric materials which have a higher dielectric permittivity. These
materials allow to achieve a high physical thickness together with a small
effective oxide thickness (EOT). The EOT is defined as the thickness of a
SiO layer with equal capacitance. For a layer of SiO
and a high-
dielectric, the EOT is
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However, the gate dielectric reliability is a crucial issue, in
particular with new materials. The parasitic tunneling current which flows
through the dielectric gives rise to wear-out which means that the blocking
capability of the dielectric is reduced, and even dielectric breakdown which
is a sudden conductance increase. It is commonly assumed that this breakdown
is caused by the gradual buildup of defects in the dielectric layer which may
be caused by anode hole injection or the release of hydrogen from the
Si-SiO interface [24]. This is especially critical for high-
dielectrics which do not form a native layer on silicon.
A. Gehring: Simulation of Tunneling in Semiconductor Devices