Field effect transistor (FET)

Definition

A field effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current. Unlike bipolar junction transistors (BJTs), which rely on the movement of both electrons and holes, FETs primarily depend on the flow of charge carriers in a semiconductor channel, making them essential for various electronic applications.

5 Must Know Facts For Your Next Test

1. FETs are known for their high input impedance, which makes them less load on preceding circuits compared to BJTs.
2. They can be used in both analog and digital circuits, providing versatility in applications ranging from amplifiers to switching devices.
3. FETs are typically less noisy than BJTs, making them favorable for high-frequency applications.
4. The most common types of FETs include MOSFETs (Metal-Oxide-Semiconductor FETs) and JFETs (Junction FETs), each with unique characteristics.
5. In FETs, the control of the output current is achieved by varying the voltage at the gate terminal, leading to energy-efficient operation.

Review Questions

• Compare the operational principles of FETs and BJTs in terms of current control and input impedance.
  • FETs operate by using an electric field to control current flow through a semiconductor channel, while BJTs rely on the movement of charge carriers involving both electrons and holes. A significant difference is that FETs have high input impedance, which means they draw very little current from the input signal. This makes FETs particularly advantageous in applications where signal integrity is crucial, as they cause minimal loading effects on preceding stages compared to BJTs.
• Discuss how gate voltage influences the performance of a FET and compare this with how base current affects a BJT.
  • In a FET, the gate voltage directly controls the conductivity of the channel between the source and drain. Increasing the gate voltage enhances channel conductivity, allowing more current to flow from drain to source. In contrast, BJTs require a base current to control larger collector-emitter currents, meaning that BJTs are more dependent on external input for operation. This illustrates how FETs provide greater efficiency and simplicity in circuit design due to their voltage-driven nature.
• Evaluate how the unique characteristics of FETs impact their application in modern electronic devices compared to BJTs.
  • FETs are increasingly favored in modern electronics due to their high input impedance, low noise levels, and ability to operate at high frequencies. These features allow for better performance in sensitive applications like RF amplifiers and digital circuits. Moreover, as technology moves towards miniaturization and integration, FETs like MOSFETs are essential in creating compact and energy-efficient devices. This trend contrasts with BJTs, which may not meet the stringent demands of contemporary electronics, highlighting the shift towards FET technology in design considerations.

Related terms

Bipolar Junction Transistor (BJT): A type of transistor that uses both electron and hole charge carriers to amplify or switch electrical signals.

Drain-Source Current: The current flowing between the drain and source terminals of a FET, influenced by the voltage applied to the gate terminal.

Gate Voltage: The voltage applied to the gate terminal of a FET, which controls the conductivity of the channel between the source and drain.