nMOSFET as Capacitor-  Working

1. Consider that the Gate Voltage applied is very negative
   1. The negative voltage at the Gate attracts holes of the p-type substrate towards the oxide interface.
   2. As the positive charge carriers (holes) are accumulating at the oxide interface, we say that the MOS Transistor is operating in the Accumulation region
   3. The two terminal device (Gate - Substrate) is now acting as Capacitor having  unit area capacitance of Cox. Note that this is due to the presence of a thin oxide layer tox that separates the Gate and the Substrate.
2. Now let us consider the case where Gate Voltage is increasing
   1. The number Holes at the interface reduce
   2. Depletion region forms
   3. The Transistor enters into the weak inversion region
3. The Capacitance now is a series combination of two capacitances - the Oxide Capacitance Cox and the Depletion Capacitance Cdep.
   1. The Gate voltage applied exceeds the Threshold Voltage 
   2. Inversion layer is formed at the Oxide-Silicon interface and is sustained. 

The unit area capacitance is Cox again
Figure 27 shows a plot of CGS vs VGS

Figure 27: CGS vs VGS

Note:

As the size of the Transistor reduces beyond 20 nm, the geometry of the MOSFET device is changed, so that the device still works as a MOS transistor.

One such change is the transfer from 2-D structure to 3-D structure, such a device is called FinFET.

This device performance is good as the length of the channel falls below 20 nm. It has been found that the I-V characteristics of the FinFET  are almost square - law, this resembles a large signal model again.

The FinFET consists of:

1. Vertical Silicon Fin

2. Oxide Dielectric

3. Poly Silicon or Metal Gate

Gate voltage controls the current flow from one fin to the other

Figure 28 shows the structure of FinFET

Figure 28: FinFET Structure

Figure 29 below shows the top view of the FinFET, which looks similar to that of a planar  MOSFET

Figure 29: Top View of the FinFET