To use MOSFET device as a Capacitor:
Let us use nMOSFET device here for discussion.
Use the Figure 26 for the working of nMOSFET as a Capacitor.
Figure 26: MOSFET(n) as Capacitor - Accumulation region
The following points to be noted:
The Source, Drain and the Bulk/Body terminals are grounded
The Gate voltage applied is variable
For Gate Voltage less than the Threshold Voltage, the transistor operates in the sub-threshold region
For Gate Voltage equal to or greater than the Threshold Voltage, the inversion layer starts developing in the transistor.