nMOSFET as Capacitor- Working

Introduction of Field Effect Transistors Distinction between a BJT and FET Junction Field Effect Transistor (JFET) N Channel JFET Summary of JFET Operation Mutual or Transfer Characteristic of n-channel JFET Amplification Factor 𝝁 Definition of Parameters p-Channel JFET

Introduction of Metal Oxide Semiconductor Field Effect Transistor - Mosfet Terminals in MOSFET Physical structure of MOSFET MOSFET Working Operation Summary of Operation MOSFET - N Type MOSFET Operation How does Drain current flow MOSFET operation More about Threshold Voltage Current vs Voltage Characteristics - Derivation Non Ideal Current -Voltage Effects or Second Order Effects Short Channel Effects MOSFET Device Capacitances MOSFET Large and Small signal Model pMOSFET Small SIgnal Model To use MOSFET device as a Capacitor nMOSFET as Capacitor- Working

Single Stage Amplifiers Common Source CS Stage with Diode-Connected Load Circuit to measure the Equivalent Resistance Common Drain Stage or Source Follower Common Gate Amplifier Large Signal Behavior Calculation of Input Resistance of a Common Gate Stage

Inverter with Enhancement n- Type MOS as Load Inverter with Linear Enhancement Type Load

nMOSFET as Capacitor- Working

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nMOSFET as Capacitor- Working

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.
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
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: