Operation

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

Operation

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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

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Operation

Field Effect Transistors

Field Effect Transistors

Metal Oxide Semiconductor Field Effect Transistor - Mosfet

Metal Oxide Semiconductor Field Effect Transistor - Mosfet

Amplifiers

Amplifiers

Inverters

Inverters

Operation