JFET is a:

Explanation:

JFET (Junction Field Effect Transistor)

Definition: A Junction Field Effect Transistor (JFET) is a type of field-effect transistor that controls the electrical conductivity of a channel using an electric field. The JFET is a voltage-controlled device where the current flow through the channel is modulated by the voltage applied to the gate terminal.

Working Principle: The JFET operates with three terminals: the source (S), the drain (D), and the gate (G). The channel through which the current flows is between the source and the drain. The gate terminal, which is reverse-biased, controls the width of the channel by varying the voltage applied to it. As the gate-to-source voltage (V_GS) changes, it alters the depletion region within the channel, thus controlling the current flow from the source to the drain (I_DS).

When a negative voltage is applied to the gate (in the case of an n-channel JFET), it increases the depletion region, narrowing the channel and reducing the current flow. Conversely, when the gate voltage is less negative, the depletion region is narrower, allowing more current to flow. This voltage control of the channel current is what characterizes the JFET as a voltage-controlled device.

Advantages:

• High input impedance, which means very little current is drawn from the input signal source, making it ideal for use in amplifiers.
• Low noise generation due to the absence of minority carrier injection, which is common in bipolar junction transistors (BJTs).
• Simple construction and ease of integration into circuits.

Disadvantages:

• Relatively lower gain compared to other transistor types, such as BJTs.
• Limited frequency response for high-frequency applications.

Applications: JFETs are commonly used in amplifier circuits, impedance buffering, analog switches, and as electronic gates due to their high input impedance and low noise characteristics.

Correct Option Analysis:

The correct option is:

Option 2: Voltage-controlled device.

This option correctly describes the nature of a JFET. The JFET is controlled by the voltage applied to its gate terminal, which modulates the current flow through the channel between the source and the drain. The voltage at the gate controls the width of the depletion region in the channel, thereby controlling the current flow, making it a voltage-controlled device.

Important Information

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

Option 1: Current-controlled device.

This option is incorrect. A current-controlled device is one where the output current is controlled by the input current. This characteristic is typical of a Bipolar Junction Transistor (BJT), where the base current controls the collector current. In contrast, a JFET is controlled by the gate voltage, not the current.

Option 3: A complementary device.

This option is incorrect as it does not describe the fundamental operation of a JFET. While JFETs can be used in complementary pairs (n-channel and p-channel), this does not define their basic mode of operation, which is voltage control.

Option 4: Low input resistance device.

This option is incorrect. JFETs are characterized by their high input impedance, not low input resistance. The high input impedance is a significant advantage, as it ensures that the device draws minimal current from the input signal source.

Conclusion:

Understanding the operational characteristics of JFETs is crucial for their effective application in electronic circuits. A JFET, as explained, is a voltage-controlled device where the gate voltage modulates the current flow through the channel. This high input impedance and low noise generation make JFETs suitable for various applications, including amplifiers and analog switches. The incorrect options highlight common misconceptions about JFETs, emphasizing the importance of distinguishing between voltage-controlled and current-controlled devices, as well as recognizing the significance of input impedance in transistor operation.