nMOSFET as Capacitor- Working
nMOSFET as Capacitor- Working
- Consider that the Gate Voltage applied is very negative
- The negative voltage at the Gate attracts holes of the p-type substrate towards the oxide interface.
- As the positive charge carriers (holes) are accumulating at the oxide interface, we say that the MOS Transistor is operating in the Accumulation region
- 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.
- Now let us consider the case where Gate Voltage is increasing
- The number Holes at the interface reduce
- Depletion region forms
- The Transistor enters into the weak inversion region
- The Capacitance now is a series combination of two capacitances - the Oxide Capacitance Cox and the Depletion Capacitance Cdep.
- The Gate voltage applied exceeds the Threshold Voltage
- 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:
-
Vertical Silicon Fin
-
Oxide Dielectric
-
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