Enhancement Type MOSFET MCQ Quiz - Objective Question with Answer for Enhancement Type MOSFET - 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.

Common-Drain Amplifier:

• A voltage buffer takes the input voltage which may have a relatively large Thevenin resistance and replicates the voltage at the output port, which has a low output resistance
• Input signal is applied to the gate
• Output is taken from the source
• To first order, voltage gain ≈ 1
• Input resistance is high
• Output resistance is low
• Effective voltage buffer stage

Common-Gate Amplifier:

• A current buffer takes the input current which may have a relatively small Norton resistance and replicates the current at the output port, which has a high output resistance
• Input signal is applied to the source
• Output is taken from the drain
• To first order, current gain ≈ 1
• – is ≈ -iout.(Current Buffer)
• Input resistance is low
• Output resistance is high
• Effective current buffer stage.

The correct answer is option 2):(Parasitic capacitors)

Concept:

At low-frequency Coupling capacitors affect the gain and effect of parasitic capacitance is negligible:

The gain at low frequency is given by:`

\(\left| {{A_{VL}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{{{f_L}}}{f}} \right)}^2}} }}\)

Where A_vL is the gain at low frequency

A_VM is the gain at mid-frequency

f_L is the lower cutoff frequency and depends on coupling and bypass capacitors

At high frequencies, the Coupling capacitor effect is negligible and parasitic capacitance reduces the gain of the amplifier.

The gain at high frequency is given by:

\(\left| {{A_{VH}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{f}{{{f_H}}}} \right)}^2}} }}\)

A_VH is the gain at high-frequency AVM is the gain at mid-frequency

f_H is the upper cutoff frequency and depends on parasitic capacitance C_gs, C_gd, and C_ds.

MOSFET parasitic capacitances are formed due to the separation of mobile charges at various regions within the structure. Parasitic Capacitances are the unwanted component in the circuit which are neglected while working at low-frequency.  

In the given circuit, the drain terminal is grounded and the output is taken from P, hence it is a common drain amplifier.

Circuit diagram of common drain amplifier

While calculating the output impedance, all the independent voltage sources must be short-circuited.

The output resistance is:

\(R_{out}={V_{out}\over I_{out}}\)

Applying KVL at the output terminal, we get:

\(V_{out}={R_s}(g_mV_{GS}+I_{out})\)............(i)

Applying KVL at the input terminal, we get:

\(V_{GS}+V_{out}=0\)

\(V_{GS}=-V_{out}\)............(ii)

Putting the value of equation (ii) in (i), we get:

\(V_{out}={R_s}(-g_mV_{out}+I_{out})\)

\(R_{out}={V_{out}\over I_{out}}={R_s\over 1+g_mR_s}\)

Dividing the numerator and denominator by g_m, we get:

\(R_{out}={V_{out}\over I_{out}}={R_s/g_m\over R_s+1/g_m}\)

R_out = \(\rm R_s||g_m^{-1}\)

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:

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

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:

The correct answer is option 2):(Parasitic capacitors)

Concept:

At low-frequency Coupling capacitors affect the gain and effect of parasitic capacitance is negligible:

The gain at low frequency is given by:`

\(\left| {{A_{VL}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{{{f_L}}}{f}} \right)}^2}} }}\)

Where A_vL is the gain at low frequency

A_VM is the gain at mid-frequency

f_L is the lower cutoff frequency and depends on coupling and bypass capacitors

At high frequencies, the Coupling capacitor effect is negligible and parasitic capacitance reduces the gain of the amplifier.

The gain at high frequency is given by:

\(\left| {{A_{VH}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{f}{{{f_H}}}} \right)}^2}} }}\)

A_VH is the gain at high-frequency AVM is the gain at mid-frequency

f_H is the upper cutoff frequency and depends on parasitic capacitance C_gs, C_gd, and C_ds.

MOSFET parasitic capacitances are formed due to the separation of mobile charges at various regions within the structure. Parasitic Capacitances are the unwanted component in the circuit which are neglected while working at low-frequency.  

The correct answer is option 4):(silicon dioxide)

Concept:

• MOSFETs or Metal Oxide Silicon Field Effect Transistors 
• The p-type semiconductor forms the base of the MOSFET.
• The two types of the base are highly doped with an n-type impurity .
• From the heavily doped regions of the base, the terminals source and drain originate.

​

• The layer of the substrate is coated with a layer of silicon dioxide for insulation.
• A thin insulated metallic plate is kept on top of the silicon dioxide and it acts as a capacitor.
• The gate terminal is get out from the thin metallic plate.

In the given circuit, the drain terminal is grounded and the output is taken from P, hence it is a common drain amplifier.

Circuit diagram of common drain amplifier

While calculating the output impedance, all the independent voltage sources must be short-circuited.

The output resistance is:

\(R_{out}={V_{out}\over I_{out}}\)

Applying KVL at the output terminal, we get:

\(V_{out}={R_s}(g_mV_{GS}+I_{out})\)............(i)

Applying KVL at the input terminal, we get:

\(V_{GS}+V_{out}=0\)

\(V_{GS}=-V_{out}\)............(ii)

Putting the value of equation (ii) in (i), we get:

\(V_{out}={R_s}(-g_mV_{out}+I_{out})\)

\(R_{out}={V_{out}\over I_{out}}={R_s\over 1+g_mR_s}\)

Dividing the numerator and denominator by g_m, we get:

\(R_{out}={V_{out}\over I_{out}}={R_s/g_m\over R_s+1/g_m}\)

R_out = \(\rm R_s||g_m^{-1}\)

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.

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

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:

The correct answer is option 2):(Parasitic capacitors)

Concept:

At low-frequency Coupling capacitors affect the gain and effect of parasitic capacitance is negligible:

The gain at low frequency is given by:`

\(\left| {{A_{VL}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{{{f_L}}}{f}} \right)}^2}} }}\)

Where A_vL is the gain at low frequency

A_VM is the gain at mid-frequency

f_L is the lower cutoff frequency and depends on coupling and bypass capacitors

At high frequencies, the Coupling capacitor effect is negligible and parasitic capacitance reduces the gain of the amplifier.

The gain at high frequency is given by:

\(\left| {{A_{VH}}} \right| = \frac{{\left| {\;{A_{VM}}} \right|}}{{\sqrt {1 + {{\left( {\frac{f}{{{f_H}}}} \right)}^2}} }}\)

A_VH is the gain at high-frequency AVM is the gain at mid-frequency

f_H is the upper cutoff frequency and depends on parasitic capacitance C_gs, C_gd, and C_ds.

MOSFET parasitic capacitances are formed due to the separation of mobile charges at various regions within the structure. Parasitic Capacitances are the unwanted component in the circuit which are neglected while working at low-frequency.  

The correct answer is option 4):(silicon dioxide)

Concept:

• MOSFETs or Metal Oxide Silicon Field Effect Transistors 
• The p-type semiconductor forms the base of the MOSFET.
• The two types of the base are highly doped with an n-type impurity .
• From the heavily doped regions of the base, the terminals source and drain originate.

​

• The layer of the substrate is coated with a layer of silicon dioxide for insulation.
• A thin insulated metallic plate is kept on top of the silicon dioxide and it acts as a capacitor.
• The gate terminal is get out from the thin metallic plate.