What is the required connection for an enhancement type N-MOSFET to function as a resistor?

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.