MOSFET Working:

Look at the Figure 6 below:

Figure 6: Showing the connection between the Source and Drain

As seen from the figure above, there are two diodes connected back to back, one at the Drain end and the other at the Source end.

Will the current flow under such conditions? Let us discuss

When the applied Drain - Source voltage is more than 0V, the DIode at the Drain end is reverse biased and as such there is no current flow

Now if we reverse the polarities, the Diode at the Source end is reverse biased and there is no current flow even now.

MOS Capacitor:

To better understand the MOSFET operation, just consider the Gate and Body i.e. remove the Source and Drain terminal.

This type of structure is represented in Figure 7 below.

Figure 7: MOS Structure without Source and Drain

As seen from this figure, the top layer is a good conductor called the Gate, the middle layer is an insulator (Silicon Dioxide) and the third layer is a doped substrate. The Substrate is either n type or p type.

This type of three layered structure resembles  a Capacitor, therefore this structure is called MOS CAPACITOR.

The MOS capacitor operation is the key in MOSFET operation.

The structure is essentially a parallel - plate Capacitor formed by the Gate, oxide as dielectric and the semiconductor (substrate).

The Gate is earlier made up of metal conductors, but later polysilicon is being used as Gate material. As the transistor size is reducing, metals are again being considered as Gate materials at sizes less than 65 nm.

The Gate capacitance Cgate is given as:

Cgate= ∈oxA / tox

Let us define Oxide Capacitance as 

COX=  ∈ox/tox

The units are F/m2 or F/𝞵m2