FET: Advantages and Disadvantages of Field Effect Transistors

Field Effect Transistors (FETs) are integral to modern electronics, providing efficient current control in circuits. Their unique properties make them suitable for a wide range of applications, from amplifiers to switching devices. However, despite their advantages, FETs also have certain disadvantages. In this guide, we will explore the main benefits and drawbacks of FETs in various applications.

What is FET?

FET is the short form of Field Effect Transistor. It is called a unipolar transistor as current flow is carried out by majority charge carriers only. A FET transistor is controlled by voltage rather than current, unlike a BJT.

A Field Effect Transistor (FET) operates by controlling the flow of current between two terminals, the source and the drain, via an electric field applied to a third terminal called the gate. Unlike a bipolar junction transistor (BJT) that relies on current control, a FET is a voltage-controlled device, where a small input voltage at the gate affects the current flow between the source and drain.

FETs have a high input impedance, making them efficient and requiring minimal input current. When a positive or negative voltage is applied to the gate (depending on whether it is an N-channel or P-channel FET), it creates an electric field that modulates the conductivity of the semiconductor material between the source and drain, thus allowing or stopping the flow of current.

The gate in a FET is separated from the conducting channel by a thin insulating layer, typically made of silicon dioxide in MOSFETs (Metal-Oxide-Semiconductor FETs), which prevents direct current from flowing into the gate. This design enables precise control of current without significant power loss.

The modulation of the channel’s conductivity, which controls current flow, enables FETs to serve as switches or amplifiers in digital and analog circuits. Their efficiency and fast switching speeds make them ideal for use in high-speed electronics, such as processors and memory devices, where they perform logic operations and signal amplification with minimal power consumption.

Based on the principle of operation and construction, FETs are classified as follows:

• JFET - Junction Field Effect Transistor
• MESFET - Metal Semiconductor Field Effect Transistor
• HEMT - High Electron Mobility Transistor
• MOSFET - Metal Oxide Semiconductor Field Effect Transistor

Advantages of FET

Following are some of the benefits of FET:

1. It offers high input impedance, reducing the load on preceding stages and allowing minimal signal interference.
2. Due to their construction, FETs produce less noise than BJTs, which is advantageous in audio and low-signal applications.
3. It offers higher efficiency as compared to BJT.
4. Operating frequency range is higher compared to conventional BJT transistor.
5. FETs consume less power compared to bipolar junction transistors (BJTs), making them suitable for battery-powered and portable devices.
6. FETs are generally more thermally stable than BJTs, leading to less variation in performance due to temperature changes.
7. FETs are excellent for high-speed applications, thanks to their ability to switch on and off rapidly, making them ideal for digital circuits.

Disadvantages of FET

Following are some of the drawbacks of FET:

1. It has a relatively lower gain-bandwidth product compared to BJT.
2. Transconductance is low and hence voltage gain is low.
3. FET has slower switching times compared to BJT. The internal junction capacitance of FET is responsible for high delay times.
4. FET performance degrades as frequency increases. This is due to the feedback by internal capacitance.
5. They are more costly compared to junction transistors.
6. FET of MOSFET type is very susceptible to overload voltages, hence special handling is required during installation.
7. In MOSFET, the layer between Gate and Channel is very fragile which is vulnerable to electro-static damage during installation. Once it is installed in a well-designed circuit, there is no issue found.

Conclusion

FETs offer numerous benefits in terms of efficiency and control, making them invaluable in electronics. Nevertheless, they are not without drawbacks, especially in terms of high-frequency performance and sensitivity to overload. Understanding these aspects will help you evaluate when and where to effectively use FETs in your projects.

FAQs on FET: Questions and Answers for Interview

Question-1: What is a Field Effect Transistor (FET)?

Answer-1: A Field Effect Transistor (FET) is a type of transistor that uses an electric field to control the flow of current. FETs are voltage-controlled devices, meaning that the output current is controlled by the voltage applied to the gate terminal. Common types of FETs include Junction FETs (JFETs) and Metal-Oxide-Semiconductor FETs (MOSFETs).

Question-2: What are the advantages of FETs?

Answer-2: FETs offer high input impedance and low power consumption, making them ideal for sensitive applications. They exhibit better thermal stability, lower noise levels, and simplicity in circuit design, which enhances overall efficiency.

Question-3: What are the disadvantages of FETs?

Answer-3: FETs typically provide lower voltage gain compared to BJTs and are sensitive to voltage changes. They may have limited frequency response, are susceptible to damage from static electricity, and can be more expensive, impacting cost-effectiveness in some applications.

Question-4: How does a FET work?

Answer-4: A FET operates by applying voltage to the gate terminal, which creates an electric field that controls the flow of current between the source and drain terminals. In a MOSFET, for instance, the gate is insulated from the channel, and the applied voltage induces a conductive channel that allows current to flow when the gate voltage exceeds a certain threshold.

Question-5: What are common applications of FETs?

Answer-5: FETs are widely used in various applications which include Amplifiers, Switching devices, Voltage controlled oscillators (VCOs), Analog signal processing, Sensors, analog to digital converters (ADCs) etc.

Question-6: What are the different types of FETs?

Answer-6: The main types of FETs include:

• Junction FET (JFET): Controlled by reverse biasing the gate.
• Metal-Oxide-Semiconductor FET (MOSFET): Includes enhancement-mode and depletion-mode types, with an insulating layer between the gate and channel.
• Insulated Gate Bipolar Transistor (IGBT): Combines the features of BJTs and MOSFETs, suitable for high-voltage applications.

Question-7: How do FETs compare to BJTs?

Answer-7: FETs and BJTs serve similar functions but operate differently. FETs are voltage-controlled devices with high input impedance and lower power consumption, making them suitable for high frequency and low power applications. In contrast, BJTs are current controlled devices, providing higher gain and better performance in amplification tasks but generally requiring more power and having lower input impedance.