Types of FET MCQ Quiz - Objective Question with Answer for Types of FET - Download Free PDF

Explanation:

Enhancement Type N-MOSFET as a Resistor

Definition: An enhancement type N-MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a semiconductor device that operates as a switch or amplifier based on the control of charge carriers (electrons for N-MOSFET) in its channel. When configured appropriately, it can also function as a resistor under specific conditions. This behavior is particularly useful in analog circuits and applications requiring variable resistance.

Correct Option:

The correct option is:

Option 3: Drain connected to source.

When the drain of an enhancement type N-MOSFET is connected to the source, the device operates in a specific mode where it can function as a resistor. This configuration is achieved by ensuring that the gate is biased appropriately to allow a conductive channel to form. The flow of current through this channel mimics the behavior of a resistor, as the MOSFET offers a controllable resistance depending on the voltage applied to the gate.

Working Principle:

To understand how an enhancement type N-MOSFET functions as a resistor when the drain is connected to the source, consider the following:

1. Gate Voltage Control: The gate voltage (V_GS) determines whether the channel in the MOSFET is conductive or non-conductive. For an enhancement type N-MOSFET, a positive gate voltage is required to induce a conductive channel of electrons between the source and drain.
2. Drain-Source Connection: When the drain is connected directly to the source, the potential difference between these two terminals is zero (V_DS = 0). This eliminates the directional flow usually associated with the MOSFET's switching operation, leaving behind the inherent resistance of the channel as the dominant factor.
3. Resistive Behavior: The channel resistance is controlled by the gate voltage. As V_GS increases, the channel becomes more conductive, reducing resistance. Conversely, lowering V_GS increases the channel resistance. This behavior allows the MOSFET to function as a variable resistor.

Advantages:

• Provides a controllable resistance for applications requiring variable resistance.
• Compact size and integration capability in circuits compared to traditional resistors.
• Useful in analog applications such as voltage-controlled resistors and signal modulation.

Disadvantages:

• Limited range of resistance values compared to mechanical or discrete resistors.
• Requires precise gate voltage control to achieve desired resistance values.
• Susceptible to temperature variations affecting resistance characteristics.

Applications:

• Voltage-controlled resistors in analog circuits.
• Signal processing and modulation systems.
• Current limiting and protection circuits.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Gate connected to source.

This configuration does not allow the MOSFET to function as a resistor. When the gate is connected to the source, the gate-source voltage (V_GS) becomes zero. For an enhancement type N-MOSFET, a positive V_GS is necessary to induce a conductive channel. Without this voltage, the MOSFET remains in the cutoff region and does not conduct, making it non-functional as a resistor.

Option 2: Gate connected to drain.

In this configuration, the gate-source voltage (V_GS) becomes equal to the drain-source voltage (V_DS). While this can cause the MOSFET to operate in certain regions, it does not provide the desired resistive behavior for the device. Instead, this setup can lead to unpredictable operation or biasing conditions, making it unsuitable for use as a resistor.

Option 4: Source open-circuited.

An open-circuited source results in no current flow through the MOSFET, as the source is one of the essential terminals for current conduction. Without a source connection, the device cannot operate as a resistor or perform any other function. This configuration is invalid for any practical application.

Option 5: Incorrect or irrelevant setup.

This option is not applicable or valid in the context of configuring an enhancement type N-MOSFET as a resistor. It does not correspond to any practical or theoretical method of operation.

Conclusion:

To configure an enhancement type N-MOSFET as a resistor, connecting the drain to the source is the correct setup. This allows the device to exhibit resistive behavior controlled by the gate voltage. Understanding the operational principles and limitations of this configuration is crucial for its effective use in analog circuits and applications requiring variable resistance.

Explanation:

Correct Option Analysis:

The correct option is:

Option 4: 1.8 mA

We are given an N-channel E-MOSFET with the following parameters:

• ID(ON) = 5 mA at VGS = 10 V
• VGS(th) = 5 V

We need to calculate its drain current (ID) for VGS = 8 V.

The drain current for an E-MOSFET in the saturation region can be calculated using the following equation:

ID = ID(ON) * ((VGS - VGS(th)) / (VGS(ON) - VGS(th)))^2

Given:

• ID(ON) = 5 mA
• VGS(ON) = 10 V
• VGS(th) = 5 V
• VGS = 8 V

Substitute the given values into the equation:

ID = 5 mA * ((8 V - 5 V) / (10 V - 5 V))^2

Calculate the terms inside the parentheses first:

(8 V - 5 V) = 3 V

(10 V - 5 V) = 5 V

Now, substitute these values back into the equation:

ID = 5 mA * (3 V / 5 V)^2

Simplify the fraction:

ID = 5 mA * (0.6)^2

Calculate the square of 0.6:

(0.6)^2 = 0.36

Now, multiply this by 5 mA:

ID = 5 mA * 0.36

ID = 1.8 mA

Therefore, the drain current for VGS = 8 V is 1.8 mA, making option 4 the correct answer.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 3.2 mA

Using the same method, if we substitute the values into the equation:

ID = 5 mA * ((8 V - 5 V) / (10 V - 5 V))^2

ID = 5 mA * (3 V / 5 V)^2

ID = 5 mA * (0.6)^2

ID = 5 mA * 0.36

ID = 1.8 mA

Clearly, the calculation shows that the correct drain current is 1.8 mA and not 3.2 mA, so option 1 is incorrect.

Option 2: 4 mA

Again, using the same method:

ID = 5 mA * ((8 V - 5 V) / (10 V - 5 V))^2

ID = 5 mA * (3 V / 5 V)^2

ID = 5 mA * (0.6)^2

ID = 5 mA * 0.36

ID = 1.8 mA

The calculation confirms that 1.8 mA is the correct value, so option 2 is incorrect as well.

Option 3: 2.6 mA

Using the same method:

ID = 5 mA * ((8 V - 5 V) / (10 V - 5 V))^2

ID = 5 mA * (3 V / 5 V)^2

ID = 5 mA * (0.6)^2

ID = 5 mA * 0.36

ID = 1.8 mA

The calculation clearly shows that 2.6 mA is not the correct drain current, thus option 3 is also incorrect.

Conclusion:

Understanding the operation of an N-channel E-MOSFET and the application of the drain current equation are crucial for correctly identifying the drain current for a given gate-source voltage. By carefully substituting the given values and performing the calculations, we have determined that the correct drain current for VGS = 8 V is 1.8 mA. This confirms that option 4 is the correct answer.

Detailed Explanation of the JFET Concept:

The Junction Field-Effect Transistor (JFET) is a type of field-effect transistor that is widely used in electronic circuits. It is a voltage-controlled device with three terminals: the Source, Gate, and Drain.

• Drain Current Control: The drain current (I_D) in a JFET is controlled by the voltage applied to the gate (V_GS) and the resulting channel width. When a reverse bias voltage is applied to the gate-source junction, the width of the conducting channel between the source and drain changes, thus controlling the current flow through the device.
• Gate-Source p-n Junction Biasing: - In a JFET, the gate-source p-n junction is always reverse biased during normal operation. This reverse biasing causes a depletion region to form, which modulates the conductivity of the channel. - Forward biasing the gate-source p-n junction is not a typical mode of operation for a JFET as it would lead to excessive current flow and loss of control over the drain current.
• Voltage-Controlled Device: The JFET is classified as a voltage-controlled device because the output current (drain current) is controlled by the input voltage (gate-source voltage) rather than the input current. This characteristic makes JFETs useful in applications requiring high input impedance and low noise.

Correct Option Explanation:

Option 2 states that "Gate-source p-n Junction is always forward biased," which is incorrect for a JFET. This statement contradicts the fundamental operating principle of a JFET, where the gate-source p-n junction is always reverse biased to control the drain current. Forward biasing the gate-source junction would compromise the device's ability to control current flow and is not a typical operating condition for JFETs.

Therefore, option 2 is the correct answer as it is the statement that is not true for a JFET.

Concept

Operating Regions of JFET

where, VDS = Drain to source voltage

VGS = Gate to source voltage

VT = Threshold voltage

Explanation

The conditions for achieving the minimum ON resistance in the n-channel JFET are:

• It is achieved when the JFET operates in the ohmic region (triode region).
• The JFET channel resistance is lowest when V_GS = 0 because the channel is fully open.
• The drain-source voltage V_DS must also be low, ideally nearly 0V, to avoid moving into the saturation or pinch-off regions.

VGS = 0V and  VDS = 0V keep the device in the ohmic region. This is the condition for achieving minimum ON resistance.

​A MOSFET is also known as Insulated-Gate JFET.

Consider the n-MOS shown below:

• If we apply a potential source between the drain and the source, no current will flow as there is no conducting path between the drain and the source.
• To provide the conducting path between the drain and the source, we apply a positive potential at the gate.
• On applying positive potential, there is an induced electric field at the semiconductor surface below the oxide, trapping the electrons (minority carrier of the p-type substrate). This results in the formation of a conducting path channel between the drain and the source.
• As there exists an oxide layer between Gate and channel which is a complete insulator. Hence it is called insulated gate FET ( IG FET ).

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

MOSFET transistor is a semiconductor device which is used for amplifying and switching electronic signals in electronic devices.

MOSFET is of two types:

1. Enhancement MOSFET:

Concept:

Small signal current gain is defined in common drain amplifier as

\(\rm A_F = \frac{I_s}{I_g} = \frac{Source \ current}{Gate \ current}\)

For FET, I_g = 0

∴ \(\rm A_i = \frac{I_s}{0}\) = Infinite

The correct answer is option 1):(N type; P type)

Concept:

• The schematic of an n-channel JFET along with its circuit symbol is shown in Figure
• The n-channel JFET has its major portion made of n-type semiconductors.
• The mutually-opposite two faces of this bulk material are from the source and the drain terminals.
• There are two relatively-small p-regions embedded into this substrate which are internally joined together to form the gate terminal
• Thu the source and the drain terminals are of n-type while the gate is of p-type.
• For n-JFET, the channel is a N-type channel and gates are P type
• Due to this, two pn junctions will be formed within the device, whose analysis reveals the mode in which the JFET works

Concept:

The drain current in an N-channel JFET is given by:

\(I_D = I_{DSS}({1-{V_{GS}\over V_p}})^2\)

where, I_D = Drain current

I_DSS = Drain current when the gate to the source is equal to zero

V_GS = Gate to source voltage

V_P = Pinch-off voltage

Calculation:

Given, IDSS = 12 mA

VP = −7V, VGS = −3.5V

\(I_D = 12({1-{-3.5\over -7}})^2\)

\(I_D = 12({1-{1\over 2}})^2\)

I_D = 3 mA

JFET:

• It is affiliated as junction field effect transistor
• The types of JFET are n-channel FET and p-channel FET
• A p-type material is added to the n-type substrate in n-channel FET, whereas a n-type material is added to the p-type substrate in p-channel FET.

​

• JFET is a three terminal device and since gate voltage controls the drain current, JFET is called as voltage controlled device.
• JFET’s have only depletion mode of operation.
• When there is no potential applied between gate and source terminals and a potential V_DD is applied between drain and source, then a current I₀ flows from drain to source terminals at its maximum as the channel width is more.
• When the voltage applied between gate and source terminal V_GG is reverse biased, this decreases the depletion width, thus the layers grows and the cross section of the channel decreases and hence the drain current I_D also decreases.
• Thus drain current in JFET is controlled by changing the channel width.
• Gate source p-n junction is always reverse biased because if it is forward then all the channel current will flow to the Gate and not to the source, ultimately damaging JFET.

The correct answer is option 3):unipolar

Concept:

• JFET is a unipolar-transistor, which acts as a voltage controlled current device and is a device in which current at two electrodes is controlled by the action of an electric field at a p-n junction.
• A JFET, or junction field-effect transistor, or JUGFET, is a FET in which the gate is created by reverse-biased junction

​

• JFET is a three terminal device and since gate voltage controls the drain current, JFET is called as voltage controlled device.
• JFET’s have only depletion mode of operation. When there is no potential applied between gate and source terminals and a potential
• V_DD is applied between drain and source, then a current I0 flows from drain to source terminals at its maximum as the channel width is more.
• When the voltage applied between gate and source terminal VGG is reverse biased, this decreases the depletion width, thus the layers grows and the cross section of the channel decreases and hence the drain current ID also decreases.
• Thus drain current in JFET is controlled by changing the channel width. Gate source p-n junction is always reverse biased because if it is forward then all the channel current will flow to the Gate and not to the source, ultimately damaging JFET.

MOSFET:

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.

It is a majority carrier device and also called as Voltage controlled current device (VGS control the current ID)

Operation of the MOSFET:

The drain characteristics or the plot between ID and VDS is shown below

The drain characteristics are shown in below tabular form.

VGS = Gate-Source voltage

ID = Drain current

VDS = Drain-Source voltage

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

MOSFET transistor is a semiconductor device that is used for amplifying and switching electronic signals in electronic devices.

MOSFET is of two types:

1. Enhancement MOSFET:

Concept:

• JFET stands for Junction Field Effect Transistor.
• A field effect transistor is a voltage controlled device i.e. the output characteristics of the device are controlled by input voltage. There are two basic types of field effect transistors:

  1. Junction field effect transistor (JFET)

  2. Metal oxide semiconductor field effect transistor (MOSFET)

• A JFET is a three terminal semiconductor device in which current conduction is by one type of carrier i.e. electrons or holes.
• The current conduction is controlled by means of an electric field between the gate and the conducting channel of the device. To control the conduction of current from the source to the drain, the gate voltage must be more negative than the source voltage.
• JEFTs are futher divided into two types that n-channel JEFT and p-channel JEFT. The three leads of a JEFT are labeled source, gate and drain. 

JFET is a three-terminal voltage controlled device. The voltage applied across the gate is used to control the current through the drain.

The gate to source voltage changes the channel width between two p regions, which ultimately controls the current flowing between drain and source terminals.