Environmental Engineering Reference
In-Depth Information
1.3.9.4
Mobility reduction due to lateral electric field
Mobility is further reduced due to the high lateral electric field. Since, at
a first approximation, the electric field is proportional to
this effect
is more pronounced in short-channel devices.
The linear relationship (1.68), which relates the drift velocity to the
electric field, no longer holds for high fields because the mobility strongly
depends on the field itself and decreases as the field increases. Specifically,
at high electric fields, the drift velocity of carriers deviates from the linear
dependency in (1.68) and even saturates. To account for this physical
phenomenon, the mobility,
in (1.69) is corrected as follows [6]
where is the effective mobility, represents the lateral field and the
term is the maximum drift velocity of the carriers. A typical value of
is in the order of a
This velocity limitation can be responsible for the saturation in MOS
transistors since a MOS can enter the active region before reaches the
value of Consequently, (1.32) must be adjusted to account for the
carrier saturation velocity.
1.3.9.5
Drain Induced Barrier Lowering (DIBL)
This effect is due to the strong lateral electric field and affects the
threshold voltage. The principal model assumes the channel is created by the
gate voltage only. Actually, a strong lateral field from the drain can also help
to attract electrons towards the surface. Strictly speaking, the drain voltage
influences the surface charge and helps the gate voltage to form the channel.
This effect is modeled with a reduction in the threshold voltage (that is, a
barrier lowering) and is also modeled by modifying (1.35) as [6], [8]
where
is
a corrective
factor
responsible for the
dependence of the
threshold voltage on
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