Difference Between BJT and FET

Introduce

The world of electronics is filled with a myriad of components that form the building blocks of modern technology. Among these components, transistors play a crucial role in amplification, switching, and signal processing. Two commonly used types of transistors are the Bipolar Junction Transistor (BJT) and the Field-Effect Transistor (FET). While both serve similar purposes, they have distinct characteristics that set them apart. Understanding the differences between BJT and FET is essential for engineers and enthusiasts alike, as it can help in choosing the right transistor for specific applications. In this article, we will delve into the dissimilarities between these two fundamental transistor types, exploring their unique features, working principles, and advantages.

What is BJT

BJT stands for Bipolar Junction Transistor. It is a type of transistor that is widely used in electronic circuits for amplification or switching purposes. The BJT consists of three layers of semiconductor material: the emitter, the base, and the collector. These layers are either P-N-P or N-P-N, depending on the type of BJT (NPN or PNP).

Figure 1: Bipolar Junction Transistor Structure Diagram

The BJT operates based on the principles of both majority and minority charge carriers. The majority charge carriers (electrons in NPN or holes in PNP) flow from the emitter to the base, and then a small portion of them continues to the collector. The flow of these charge carriers can be controlled by the current or voltage applied to the base terminal.

BJTs have three different modes of operation: active mode, saturation mode, and cutoff mode. In the active mode, the transistor acts as an amplifier, where a small input current or voltage controls a larger output current. In the saturation mode, the transistor is fully turned on, allowing a maximum current to flow through it. In the cutoff mode, the transistor is turned off, and no current flows through it.

BJTs are widely used in various applications, including audio amplifiers, power supplies, radio frequency (RF) amplifiers, digital logic circuits, and many other electronic devices.

What is FET

FET stands for Field-Effect Transistor. It is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs are voltage-controlled devices and are widely used in electronic circuits for amplification, switching, and other applications.

FETs have three terminals: the source, the drain, and the gate. The gate terminal controls the conductivity of the channel between the source and the drain. Depending on the type of FET, the channel can be formed by either majority electrons (N-channel FET) or majority holes (P-channel FET).

Figure 2: Bipolar Field Effect Transistor Diagram

FETs offer several advantages over other types of transistors, such as bipolar junction transistors (BJTs). They have high input impedance, low power consumption, and are less sensitive to variations in temperature. FETs also have faster switching speeds and are suitable for high-frequency applications.

There are different types of FETs, including Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), Junction Field-Effect Transistors (JFETs), and Insulated-Gate Bipolar Transistors (IGBTs).

Compare BJT and FET

BJT (Bipolar Junction Transistor) and FET (Field-Effect Transistor) are two different types of transistors with distinct operating principles and characteristics. Here are the key differences between BJT and FET:

Parameter	BJT (Bipolar Junction Transistor)	FET (Field-Effect Transistor)
Full form	Bipolar Junction Transistor	Field Effect Transistor
Definition	A type of transistor which uses two types of charge carriers viz. electrons and holes for conduction.	A type of transistor in which electric field is used to control the flow of current in a semiconductor.
Drive type	Current flows due to both majority and minority charge carriers.	Current flows only due to majority charge carriers.
Terminals	Emitter, Base, and Collector	Source, Drain, and Gate
PN junction	Consists of two PN junctions: emitter-base junction and collector-base junction.	Does not have PN junctions.
Control element	Current-controlled device	Voltage-controlled device
Types	NPN transistor and PNP transistor	N-channel FET and P-channel FET
Configuration	Common emitter (CE), common base (CB), and common collector (CC)	Common source (CS), common gate (CG), and common drain (CD)
Size	Larger in size and requires more space. More complicated to fabricate as an IC.	Smaller in size. Easier to fabricate as an IC.
Sensitivity	More sensitive to changes in applied voltage.	Less sensitive to variations in applied voltage.
Relationship	Linear relationship between input and output.	Non-linear relationship between input and output.
Thermal noise	More thermal noise.	Lower thermal noise.
Thermal runaway	Thermal runaway can occur.	Thermal runaway does not occur.
Thermal stability	Less thermal stability.	Good thermal stability due to the absence of minority charge carriers.
Input impedance	Low input impedance due to forward bias of input circuit.	High input impedance due to reverse bias of input circuit.
Temperature coefficient	Positive temperature coefficient.	Negative temperature coefficient.
Suitability	Suitable for low current applications.	Suitable for high current applications.
Switching speed	Low switching speed.	Higher switching speed.
Effect of radiation	Susceptible to radiation.	Relatively immune to radiation.
Gain bandwidth product	Higher gain bandwidth product.	Lower gain bandwidth product.
Minority carrier storage	Suffers from minority carrier storage effect.	Does not suffer from minority carrier storage effect.
Cost	Cheaper to manufacture.	Relatively expensive to manufacture.
Installation	Does not require special handling during installation.	Demands special handling during installation.
Applications	Used as a switch (in saturation and cut-off region) and amplifier (in active region).	Used as a switch (in Ohmic and cut-off region) and as an amplifier (in saturation region).

The choice between BJT and FET depends on the specific application requirements. BJT is often preferred for low-power and high-frequency applications, while FET, especially MOSFET, is commonly used in power electronics, digital circuits, and applications where high input impedance and low power consumption are important.

Advantages of BJT over FET

Bipolar Junction Transistors (BJTs) have advantages over Field-Effect Transistors (FETs) in certain applications. BJTs offer higher current handling capabilities, lower output impedance, lower voltage requirements, simpler biasing circuits, better linearity, and can be more cost-effective. They are suitable for power applications, driving loads directly, low voltage operation, simpler circuit design, precise signal amplification, and cost-sensitive applications.

Advantages of FET over BJT

Field-Effect Transistors (FETs) offer several advantages over Bipolar Junction Transistors (BJTs). FETs have a high input impedance, which allows for easier integration into circuits and reduces the loading effect on the preceding stages. They also have low power consumption, making them suitable for battery-powered devices. FETs have fast switching speeds, enabling efficient digital signal processing. They exhibit low noise characteristics, making them ideal for amplification applications. Additionally, FETs have a high voltage gain and good thermal stability.

Final Evaluation and Conclusion

In conclusion, the Bipolar Junction Transistor (BJT) and the Field-Effect Transistor (FET) are two distinct transistor types with their own set of characteristics and advantages. The BJT, with its current-controlled behavior and low input impedance, remains a popular choice for applications requiring high current amplification and low-frequency operation. On the other hand, the FET, with its voltage-controlled behavior, high input impedance, low power consumption, and fast switching speeds, excels in high-frequency applications, low-power circuits, and digital signal processing.

The choice between BJT and FET depends on the specific requirements of the application, taking into consideration factors such as current handling capabilities, voltage gain, power consumption, and cost-effectiveness. By understanding the differences between BJT and FET, engineers can make informed decisions to optimize the performance of their electronic designs and pave the way for technological advancements in various fields.

FAQ

1.What are the major differences between BJT and FET?

The key difference is that a BJT is a current-controlled device, while a FET is a voltage-controlled device. This distinction impacts their behavior, characteristics, and applications in electronic circuits.

2.What is the basic difference between BJT and FET?

The basic difference between a Bipolar Junction Transistor (BJT) and a Field-Effect Transistor (FET) lies in their fundamental operating principles and construction.

BJT:

A BJT is a three-layer semiconductor device consisting of two pn-junctions. It is a current-controlled device, meaning the input current controls the output current. The BJT operates by allowing a small current to control a larger current flow. It has three terminals: the emitter, the base, and the collector. The BJT can be either an NPN (negative-positive-negative) or a PNP (positive-negative-positive) transistor, depending on the type of doping used in its construction.

FET:

A FET, on the other hand, is a three-terminal semiconductor device that operates based on the voltage applied to its terminals. It is a voltage-controlled device, where the input voltage controls the output current. The FET consists of a channel made of a semiconductor material, with a gate terminal controlling the flow of current through the channel. The FET has three main types: the Junction Field-Effect Transistor (JFET), the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), and the Insulated-Gate Bipolar Transistor (IGBT).

In summary, the key difference is that a BJT is a current-controlled device, while a FET is a voltage-controlled device. This distinction impacts their behavior, characteristics, and applications in electronic circuits.

3.A junction Field-Effect Transistor(fet) has three terminals namely?

Source (S): The source terminal is the terminal from which the majority charge carriers (electrons for n-channel JFET or holes for p-channel JFET) enter the channel.

Drain (D): The drain terminal is the terminal through which the majority charge carriers exit the channel.

Gate (G): The gate terminal controls the conductivity of the channel by modulating the depletion region width. It determines the output current based on the voltage applied to it.

4.Fet is Unipolar or Bipolar?

FETs are unipolar devices because they rely on the movement of only one type of charge carrier (either electrons or holes)

5.BJT is Unipolar or Bipolar?

BJTs are bipolar devices as they involve the movement of both electrons and holes for their operation.

6.Fets meaning in hindi?

In Hindi, "FET" can be translated as "क्षेत्र प्रभाव ट्रांजिस्टर" (Kshetra Prabhav Transistor).

7.BJT meaning in hindi?

In Hindi, "BJT" can be translated as "बीजेटी" (Beejaytee).