### MOSFET Device Capacitances

The Figure 19 shows the different capacitances present in the MOSFET.

Figure 19: MOSFET Capacitance Model

As seen from this figure 19, capacitance exists between every two terminals of the device. Also it may be noted that the values of these capacitances depend on the type of bias applied.

Now let us consider the physical structure of the MOSFET device and identify the capacitances.

Figure 20 shows the presence of capacitances in the MOSFET.

Figure 20: Physical Structure of MOSFET showing Capacitances

Here:

1. C1 is the Capacitance between the Gate and the Channel.
C1= W L COX

2. C2 is the depletion capacitance between the Channel and the Substrate.
C2 = WLqsiNsub/(4F)

3. C3 and C4 are capacitances resulting from the overlap of the polysilicon with Source and Drain areas respectively. Let the overlap capacitance per unit width be defined as Cov. The units are F/m or fF/m. Therefore the Gate - Source and Gate- Drain capacitances are obtained by multiplying the Cov and W.

4. C5 and C6 capacitances are the junction capacitance between the Source and the Substrate and Drain and Substrate. This capacitance can be decomposed into two components:

1. Bottom plate capacitance of the junction Cj (Units : F/m2) and

2. Sidewall capacitance Cjsw (Units: F/m)

Note:

CJ is multiplied by the S/D area

Cjsw is multiplied by S/D perimeter

Discussion on Capacitances between terminals:

The MOSFET is OFF

CGD=CGS =Cov W

The Gate - Bulk capacitance is equal to the series combination of the Gate - Oxide capacitance and the depletion region capacitance.

The Capacitances CSB and CDB are functions of the Source and Drain voltages w.r.t the Substrate.