Difference between BJT and FET (Comparison Table)

In this article, I will explain the important difference between BJT and FET. The primary difference between BJT and FET is that the BJT (Bipolar Junction Transistor) is a current-controlled device while the FET (Field Effect Transistor) is a voltage-controlled device. However, both BJT and FET are used as electronic switches in the field of electronics engineering.

What is a BJT?

BJT is an acronym for Bipolar Junction Transistor. It is a semiconductor electronic device with three terminals and two p-n junctions.

The three terminals of the BJT are named “emitter”, “base”, and “collector”.

BJT is used in electronics for switching and amplification of electronic signals.

Types of BJT

Based on construction, BJT can be classified into the following two main types:

• NPN BJT – This type of BJT transistor is constructed by sandwiching a p-type semiconductor layer between two n-type semiconductor layers. Thus, the NPN BJT has an n-type emitter and collector, and a p-type base.
• PNP BJT – This type of BJT transistor consists of an n-type semiconductor layer sandwiched between two p-type semiconductor layers. This BJT construction has a p-type emitter and collector and an n-type base.

Applications of BJT

BJTs are widely used in numerous electronic applications. Some common applications of BJTs are listed below:

• BJT is used as an electronic switch to control the operation of an electronic circuit.
• BJT is used in signal amplifiers.
• BJT is used in oscillator circuits.

What is an FET?

FET is an acronym for Field Effect Transistor. FET is a voltage-controlled device, in which a voltage signal or an electric field controls the flow of current through the device.

An FET consists of three terminals namely, source, gate, and drain. The source and drain are the main circuit terminals and the gate is the control terminal, as the control voltage is applied.

FET is also used in various electronic circuits to perform switching and amplification functions.

Types of FET

FET can be classified into the following two main types:

• JFET – JFET stands for Junction Field Effect Transistor. It is the simplest type of FET that can operate in depletion mode only. It is mainly used as an electronic switch and a signal amplifier.
• MOSFET – MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It is another type of FET that can operate in both depletion and enhancement modes. The main benefit of the MOSFET transistor is that it does not require any input current to control the load circuit current. It is used in various applications such as amplifiers, voltage regulators, high-speed electronic switches, etc.

Applications of FET

The following are some key applications of FETs (Field Effect Transistors):

• FETs are used in voltmeters, oscilloscopes, and many other measuring devices as the input amplifier.
• FET is also used in FM radio amplifiers.
• FET is employed in TV and radio receivers.
• FETs are also used in power inverters.

Till this section of the article, we have learned about the fundamentals of BJT and FET. Let us now discuss the important differences between BJT and FET.

Difference between BJT and FET

The major differences between BJT and FET are given in the following table:

Parameter	BJT	FET
Full form	BJT stands for Bipolar Junction Transistor.	FET stands for Field Effect Transistor.
Current carriers	BJT is a bipolar device, as current conduction takes place due to both majority and minority charge carriers.	FET is a unipolar device, as current conduction is only due to the majority charge carriers.
Terminals	BJT has three terminals namely, emitter, base, and collector.	FET has three terminals namely, source, gate, and drain.
Type of device	BJT is a current-controlled device.	FET is a voltage-controlled device.
Control terminal	In BJT, the base terminal acts as the control terminal.	In FET, the gate terminal acts as the control terminal.
Control quantity	The base current controls the operation of the BJT.	The gate voltage controls the operation of the FET.
Types	BJTs are of two types namely, NPN and PNP.	FETs are of two types namely, JFET and MOSFET.
Input impedance	BJT has comparatively low input impedance.	FET has a higher input impedance than BJT.
Noise	BJT has a high noise level.	FET has very low noise levels.
Thermal stability	BJT has less thermal stability.	FET has very high thermal stability.
Gain	BJT produces higher gain at output.	FET produces lower gain at the output.
Gain bandwidth product	BJT has a high-gain bandwidth product.	The gain bandwidth product for FET is low.
Cost	BJT is relatively cheaper than FET.	FET is costlier than BJT.
Frequency response	BJT has a lower frequency response. Hence, the frequency variations can affect its performance.	FET has a higher frequency response.
Size	BJT has a relatively larger physical size.	FET has a smaller physical size.
Input and output relationship	BJT has almost a linear relationship between input and output.	FET has a non-linear relationship between input and output.
Offset voltage	BJT has an offset voltage before switching.	FET does not have any offset voltage.
Applications	BJTs are mainly used in low-current and analog electronic applications.	FETs are mainly used in high-power and analog and digital electronic applications.

Conclusion

In conclusion, the BJT and FET are two popular types of transistors used in electronic circuits for switching and signal amplification applications. The primary function of both transistors is the same i.e., switching and amplification of electronic signals. But they are absolutely different from each other in terms of construction, working, and other technical specifications. In this detailed article on the “difference between BJT and FET”, I have explained the basics of BJT and FET and their differences.