Let us discuss the operation of the MOS capacitor in the following steps. Here the type of substrate taken for discussion is p-type.

Step 1:

In the p type of semiconductor bar (substrate) the majority carriers are holes and the minority carriers are electrons. The minority carriers which are electrons are small in number. 

Figure 8 illustrates the same.

Figure 8: Illustrating the presence of Majority and Minority carriers 

Step 2:

Let us apply negative voltage to the Gate terminal.

This is shown in Figure 9 below.

Figure 9: Application of negative voltage to the Gate

As the Gate charge is negative, the holes are attracted to the Gate region and the electrons move away. 

This enhances the concentration of holes near the Gate i.e. the number of holes near the Gate are enhanced. 

Thus the negative voltage applied to the Gate causes the enhancement of holes near the Gate terminal. This is called “Accumulation” of holes.

The N - MOSFET is said to be in Accumulation mode of operation

This phenomenon is not of any use in a N MOSFET device.

Figure 10 illustrates the same

Figure 10: Accumulation Mode of Operation

Step 3:

Now let us apply a small positive voltage to the Gate. Because of this polarity the holes near the Gate interface are repelled away. Thus the region near the Gate is depleted of the Holes. Now the MOSFET is said to be operated in the Depletion Mode.

Even this phenomenon is not of any use, but the movement of some electrons towards the Gate is promising.

Figure 11: Depletion Mode of Operation

Step 4: 

As this voltage applied is made more and more positive, the electrons are attracted towards the region beneath the Gate. Similarly more and more holes are repelled away from the region beneath the Gate.

At a sufficient positive voltage, the electrons gather beneath the Gate region and make it negative. In other words the region beneath the Gate becomes “inverted” as the earlier p region now behaves like a n region.

The Gate voltage at which the Inversion layer is formed is called the Threshold Voltage VT.

The Threshold voltage VT depends on the dopant concentration in the body and the thickness of the oxide layer tOX. The threshold voltage VT is positive. 

Figure 12: MOS Capacitor operating in the Inversion region