Non Ideal Current -Voltage Effects or Second Order Effects

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

Non Ideal Current -Voltage Effects or Second Order Effects

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The following three effects are discussed in this section:

Body Effect
Channel Length Modulation and
Subthreshold Conduction

Body Effect:

In our analysis so far we have assumed the following:

The Body and the Source are connected together to the ground in nMOS Transistor

If the Body voltage VB drops below the Source Voltage, then what happens? Let us analyze.

The Source and Drain still remain reverse biased and the device works normally, but there are some changes in the characteristics of the device.

Let VS=VD=0 and VG is slightly less than the threshold voltage VTH

Under these conditions there exists a depletion layer under the Gate area but it must be noted there is no inversion layer.

Now if the Body voltage VB becomes negative more holes are attracted towards the substrate connection. This leaves a larger negative charge behind and the depletion layer becomes wider.

It may be noted here that the threshold voltage is a function of the total charge in the depletion region. This is possible because the Gate charge mirrors the charge density before the inversion layer is formed.

If the value of VB becomes negative, QD increases and VTH also increases.

This Phenomenon is called as Body Effects

$V_{TH}=V_{THO}+\gamma(\sqrt{2\varnothing_F+V_{SB}}-\sqrt{\left|2\phi_F\right|})$

Where 𝛄 represents body effect coefficient, VSB is the Source - Body potential difference.

Note:

The Body effect happens even if the Source Voltage varies with the Body Voltage.

Body effect is undesirable i.e. change in threshold voltage complicates the design of both analog and digital circuits.

Channel Length Modulation:

In the pinch off region the Channel length decreases with decrease in potential difference between the Gate and the Drain.

In other words the actual channel length is a function of VDS.

The equation below gives the relationship between the Drain current ID in saturation region to the channel length modulation coefficient 𝛌.

$I_D\approx\frac12\mu_nC_{OX}\frac WL\left(V_{GS}-V_{TH}\right)^2\left(I+\lambda V_{DS}\right)$

Subthreshold Conduction:

MOSFET is said to be ON when the voltage VGS is above the threshold Voltage VTH and turns OFF when the voltage VGS is below the threshold voltage VTH.

But when the VGS ≈ VTH, an inversion layer exists in the MOS transistor which is weak - weak inversion layer.

Because of the presence of the weak inversion layer, there is some small amount of current that flows between Source and Drain ID.

It may be noted that there is some finite current ID that flows in the device even when the voltage VGS< VTH.

This finite current ID is found to be exponentially dependent on the Gate Voltage VGS. This phenomenon is called Subthreshold Conduction.

$I_D=I_Oexp\frac{V_{GS}}{\xi V_T}$

Here the value 𝞷>1 is a non ideal factor

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Non Ideal Current -Voltage Effects or Second Order Effects

Field Effect Transistors

Field Effect Transistors

Metal Oxide Semiconductor Field Effect Transistor - Mosfet

Metal Oxide Semiconductor Field Effect Transistor - Mosfet

Amplifiers

Amplifiers

Inverters

Inverters

Non Ideal Current -Voltage Effects or Second Order Effects