Environmental Engineering Reference
In-Depth Information
Substituting the value defined in (1.32) into (1.30) gives the
current expression in the pinch-off case and results as
As mentioned above, (1.33) is valid at a first approximation. In fact,
increasing yields an increase in the pinch-off region as well as a decrease
in channel length. This effect is commonly known as channel length
modulation. To take this effect into account, a corrective term is used to
complete (1.33) which becomes
The parameter is referred to as the channel length modulation factor
and, at a first approximation,
it is
inversely proportional to the channel
length,
L.
1.3.4
Body Effect
All the equations derived above were based on the assumption that the
source and the substrate (or the bulk) were connected together. Although this
is a rather common condition, in general the voltage of these two terminals
can be different. In this event a second order effect occurs commonly
referred to as the
body effect
[6]. A different voltage between the source and
the bulk is modeled as an increase in the threshold voltage, which assumes
the following expression [6]-[8]
with being the source-bulk voltage, the threshold voltage with zero
the Fermi potential of the substrate and a constant referred to as the
body-effect constant. The Fermi potential is defined as [1]
