Environmental Engineering Reference
In-Depth Information
Each capacitive contribution has its own physical meaning that can be
understood by analyzing the detailed n-MOS cross section in Fig. 1.11.
The most important capacitor is the gate-source capacitor whose value is
given by two different terms. The first term takes into account the capacitive
effect between the gate and the channel, which is electrically connected to
the source. At a first approximation, the corresponding capacitor, is a
linear capacitor that depends on the oxide thickness as well as on the device
area. It can be demonstrated that its value is approximately given by [4], [7]
The second term that contributes to the gate-source capacitance is given by
the overlap that exists between the gate and the source n+ region. This
overlap is unavoidable and results from the fact that during the fabrication
process the doping element also spreads horizontally. Naming
the overlap
diffusion length, the resulting parasitic capacitor,
is given by
Hence, the capacitor
in Fig. 1.9 is expressed by the sum of (1.49) and
(1.50), that is
