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Mosfet can be certainly used as a capacitor. Most often Gate to channel(source/drain) or Gate to bulk capacitance of the mosfet is used. The mosfet can be used in accumulation mode(Vgs<<Vt), weak inversion(Vgs around Vt) or strong inversion mode(Vgs>>Vt). The cap value in accumulation and strong inversion is around WLCox. There is a transition region in between where in the cap value dips and rises again. This property is used in MOS varactors(variable capacitors).
mosfet can be used as a capacitor having very less capacitance value (in the range of pf.But for a stable capacitance value u have to operate it either in invertion region or in the accumulation region as in the other regions it will vary with voltage.
You can simply connect drain/source/bulk together with gate biased with a voltage to obtain the MOSFET as capacitors.
As it is mentioned, there are three regions for MOSFET to operate as capacitors. In practice, one always bias the MOSFET in saturation region to have stable capacitance values. However there are drawbacks
(1) Leakage in deep submicron
(2) Capacitance not preise, as MIM cap or PIP cap
(3) Power/Ground noise coupling
However, some designs do need these for it has comparably larger unit cap value than MIM or PIP. You can always use spice to run transient to make sure the cap value compare with an ideal cap. Most PLL use MOSFET as it requires less area than MIM or PIP or even some compensation purpose of Op-Amp.
and the worst part is the *harmonic distortion* !!! Only use mosfets as capacitors for decoupling bias lines... BTW, I find the value rather well controlled, since it depends on the Tox of the gate, which is possibly one of the tighest controlled parameter (opinion only, no backup by theory/facts).
If capacitance area is really large then one can run into pinhole problems, but this is unusual.
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