Field Effect Transistors MCQ Quiz - Objective Question with Answer for Field Effect Transistors - Download Free PDF

Explanation:

FET (Field Effect Transistor)

Definition: A Field Effect Transistor (FET) is a type of transistor that uses an electric field to control the flow of current. It is one of the most fundamental components in electronics and is widely used in amplifiers, switches, and digital circuits. The current through the FET is controlled by the voltage applied to the gate terminal, making it a voltage-controlled device.

Working Principle: The FET operates based on the principle that the voltage applied to the gate terminal creates an electric field, which controls the conductivity of the channel between the source and drain terminals. By varying the gate voltage, the current flowing through the channel can be modulated.

In an n-channel FET, a positive gate voltage enhances the conductivity of the channel, allowing more current to flow. Conversely, in a p-channel FET, a negative gate voltage enhances conductivity. The ability to control current with voltage makes FETs highly efficient and versatile in various applications.

Advantages:

• High input impedance, resulting in minimal loading on preceding circuits.
• Low power consumption due to the absence of a direct current path between the gate and the source.
• Compact size and compatibility with integrated circuit technology.
• Wide range of applications, including amplifiers, oscillators, and digital logic circuits.

Disadvantages:

• Susceptibility to damage from static electricity due to the high input impedance.
• Limited output current capability compared to bipolar junction transistors (BJTs).
• Temperature sensitivity, which can affect performance in extreme conditions.

Applications:

• Used in amplifiers to boost weak signals in audio, radio, and communication systems.
• Serves as a switch in digital circuits, enabling or disabling current flow based on logic levels.
• Utilized in voltage regulators and power management circuits for efficient energy control.
• Commonly employed in radio frequency (RF) applications due to their high-frequency response.

Correct Option Analysis:

The correct option is:

Option 4: Voltage controlled device

FETs are classified as voltage-controlled devices because the current flowing through the device (between the source and drain terminals) is controlled by the voltage applied to the gate terminal. This characteristic differentiates FETs from current-controlled devices like bipolar junction transistors (BJTs), where the base current controls the collector current. The voltage-controlled nature of FETs allows for efficient operation with minimal power consumption, making them suitable for a wide range of applications, from analog signal amplification to digital switching.

Additional Information

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

Option 1: Current controlled device

This option is incorrect because FETs are not current-controlled devices. In a current-controlled device like a BJT, the collector current is directly dependent on the base current. However, in FETs, the current flow is controlled by the voltage applied to the gate terminal, not by any current flowing into the gate.

Option 2: Magnetic device

This option is incorrect as FETs do not operate based on magnetic principles. Magnetic devices typically involve the use of magnetic fields for operation, such as transformers or inductors. FETs, on the other hand, rely on electric fields to control current flow.

Option 3: Power controlled device

This option is incorrect because the term "power controlled device" does not accurately describe the operation of FETs. While FETs are used in power electronics for switching and amplification, their operation is fundamentally based on voltage control rather than power control. Power control is a broader term that applies to various devices and systems, not specifically to FETs.

Option 5: (Not specified in the question)

As no specific description is provided for this option, it cannot be considered a valid choice for the classification of FETs.

Conclusion:

FETs are voltage-controlled devices that utilize an electric field to modulate the flow of current. This characteristic makes them highly efficient and versatile components in modern electronic circuits. Understanding the distinction between voltage-controlled and current-controlled devices is essential for selecting the appropriate component for a given application. The incorrect options highlight the importance of accurately identifying the operational principles of electronic devices to avoid misconceptions.

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.

Metal Oxide Silicon Field Effect Transistors (MOSFET)

• It is a voltage-controlled three-terminal device that is used for switching and amplification purposes.
• The terminals are the drain, gate, and source.

Transfer characteristics of power MOSFET

The transfer characteristics of power MOSFET, shows the variation of drain current (I_D) as a function of gate-source voltage (V_GS).

Concept

The forward current gain (α) of a power transistor is defined as:

\(α ={I_C\over I_E}\)

where, I_C = Collector current

I_E = Emitter current

For power transistors, the forward current gain (α) is typically high, usually in the range of 0.95 to 0.99. This indicates that most of the emitter current flows to the collector, with very little loss to the base current.

\(β ={α\over 1-α}\)

where β is the common-emitter current gain, having α close to 1 ensures that power transistors operate efficiently with high current gain.

Thus, the correct approximate value of forward current gain for power transistors is 0.95 to 0.99.

Concept

The drain current in a JFET is given by:

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

where, I_D = Drain current

I_DSS = Saturation drain current

V_GS = Gate -source voltage

V_P = Pinch off voltage

Calculation

Given, V_p = - 8 V

IDSS = 30 mA

VGS = - 4 V

\(I_D=30({1-{-4\over -8}})^2\)

I_D = 7.5 mA

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

Three regions of MOSFET operation (nMOS) are:

Cut off region:

VGS < Vth

ID = 0

VGS = Gate to source voltage

Vth = Threshold voltage

Linear/ Ohmic/ Triode region:

VGS > Vth

VDS < VGS – Vth

The current equation for a MOSFET in the linear region is given by:

\({{I}_{D}}={{\mu }_{n}}{{C}_{ox}}\frac{W}{L}\left\{ \left( {{V}_{GS}}-{{V}_{th}} \right){{V}_{DS}}-\frac{V_{DS}^{2}}{2} \right\}\)

Saturation region:

VGS > Vth

VDS > VGS – Vth

The current equation for a MOSFET in the saturation region is given by:

\({{I}_{D}}=\frac{1}{2}{{\mu }_{n}}{{C}_{OX}}\frac{W}{L}{{\left( {{V}_{GS}}-{{V}_{th}} \right)}^{2}}\)

Analysis:

At the edge of saturation:

\({V_{DS}} = {V_{GS}} - {V_T}\)

V_DS = 2 – (6)

V_DS = - 4 Volts

This is explained with the help of the V-I characteristics as shown:

Concept:

1) The thermal runway is not possible in FET because as the temperature of the FET increases, the mobility decreases, i.e. if the Temperature (T) ↑, the carries Mobility (μn or μ­p) ↓, and Ips↓

2) Since the current is decreasing with an increase in temperature, the power dissipation at the output terminal of a FET decreases or we can say that it’s minimum.

So, there will be no Question of thermal Runway at the output of the FET.

• The thermal runaway takes place in a BJT.
• Thermal Runway in BJT is a process of self-damage of BJT because of overheating at the collector junction due to an increase in Ic with Ico
• If T↑, then Ico (Reverse separation current) ↑, which results in an increase in the collector current, i.e. Ic ↑.
• Power dissipation at the collector junction increases in the form of heat which again raises the temperature and the cycle continues.
• If the above cycle becomes repetitive then the collector junction gets overheated and thereby thermal runway takes place.

As shown in the above figure in a power MOSFET, pinch-off occurs when V_DS = V_GS - V_T

Where, V_DS = Drain to source voltage

​V_GS = Gate to source voltage

V_T = Threshold voltage

• In power MOSFET when V_DS < V_GS - V_T, then power MOSFET works in the triode region.
• In power MOSFET when V_DS > V_GS - V_T, then power MOSFET works in the saturation region.

• FET is a voltage-driven/controlled device, i.e. the output current is controlled by the electric field applied.
• The current through the two terminals is controlled by a voltage at the third terminal (gate).
• It is a unipolar device (current conduction is only due to one type of majority carrier either electron or hole)
• It has a high input impedance.

• For a FET, either

  \({I_D} = {I_{{D_{SS}}}}{\left( {1 - \frac{{{V_{GS}}}}{{{V_P}}}} \right)^2}\)in case of JFET

  or

  \({I_D} = K{\left( {{V_{GS}} - {V_T}} \right)^2}\)in case of MOSFET

  So, a FET is a voltage-controlled current source.

The difference between FET and BJT is explained in the following table: