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floating gate capacitor

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harry456

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Why can structure (d) be used as a floating capacitor but c not? An why only when it's on strong inversion mode? What property allows a gate mos-capacitor to be used as a floating cap?

gatecaps.PNG

The thin-oxide capacitive structures available in a p-well CMOS technology are
summarized in Figure 3.14. The first two, Figures 3.14(a) and (b), must operate in
accumulation, whereas the last two can be biased in either accumulation or strong
inversion. The device in Figure 3.14(a) can act as a floating capacitor, but the one in
Figure 3.14(b) is an ac grounded capacitor since the n substrate is connected to the power
supply. The device in Figure 3.14(c) can be shielded from substrate noise by applying
a fixed potential to the well, whereas the one in Figure 3.14(d) can act as a floating
capacitor when biased in strong inversion.
Click to expand...
 

The property is the gate capacitance during commutation. The configuration with the least accumulation of charge is (d) which has the least substrate capacitance to B or S is best for floating capacitance.
 

What do you mean with "commutation"?

Structure (a) is in well to but can be used as a floating capacitor, too. (why? )
The device in Figure 3.14(a) can act as a floating capacitor
Click to expand...

Is structure (c) less suited to be used as floating capacitor because of its p-well (and the resulting capacitance to the substrate) -- how does this additional capacitance effect the floating?
 

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