Difference between BJT and FET Transistors

What Is The Difference between BJT and FET Transistors? . Both BJTs and FETs are transistors widely used in different types of electronic applications. Go through this article to find out the important features of BJTs and FETs and how they differ from each other in the way they function.

Transistors are the backbone of modern electronic circuits, enabling amplification, switching, and signal modulation. Among the various types, Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs) are the most commonly used. But what sets these two apart? This article provides a deep dive into the differences, principles of operation, applications, and practical considerations to help you better understand BJT vs. FET transistors.

What Is a BJT?

BJT stands for Bipolar Junction Transistor . It is three terminal semiconductor transistor . Three terminals are Emitter , Base , Collector .The emitter is heavily doped , Base is middle layer and the collector is lightly doped . These three terminal combine to form two P-N junction. These three regions are formed by doping at different level . BJT can be used as amplifier to amplify the signals and as switch in circuit .BJT is current controlled device . BJT can be further classified in two types .

NPN BJT

PNP BJT

In NPN BJT emitter and collector is N type and base is P type and in PNP BJT emitter and collector is P type and base is N type. In NPN transistor current flows from base to emitter and in PNP transistor current flows from emitter to base .BJT is three terminal device so we can configure it in three ways as following

Common Base Configuration : In this base act as common terminal between input and output . This configuration has voltage gain but have no current gain.
Common Emitter Configuration : In this emitter is common between input and output. This configuration has both voltage gain and current gain.
Common Collector Configuration : In this collector act as common terminal between input and output port . It has no voltage gain but have current gain.

Advantages of BJT

BJT have high current gain.
BJT can operate in any power device .
Gain bandwidth is large in BJT.
In high frequency BJT perform very well.
BJT have low forward voltage and better voltage gain .

Disadvantage of BJT

BJT required large chip area in fabrication .
Power loss is high in BJT.
It is very temperature sensitive.
Noise produced is large.
It is more sensitive to radiations.

Applications of BJT

It is used in Multivibrator.
It is used as an amplifier.
It is used in oscillator.
Used in timer and time delay circuit.
Used in electronic switching circuit.

What Is a FET?

FET stands for Filed Effect Transistor . It is three terminal semiconductor device . It have Source , Drain , Gate and Body bit mostly body is sorted along with this it have a channel through which current flows from drain to source . It is unipolar because only majority charge carriers are responsible for flow of current .FET can be used as amplifiers , switches or voltage controlled resistor. Based on the type of channel FET can be classified as

N Channel FET : In this channel is of N type

P Channel FET : In this channel is of P type

Symbol of N-channel FET and P-channel FET

Read More : Analog to Digital Converters (ADCs)

FET are of two types :

JFET : JFET stands for Junction Field Effect Transistor . It is three terminal device
MOSFET : MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor . It is four terminal device.

Advantage of FET

FET have high input impedance.
It produce less noise.
It is less sensitive to radiations.
Higher operating frequency range.
It is fabricated in less chip area.

Disadvantage of FET

Following are the some of the disadvantage of FET

FET are costly.
FET have low transconductance.
In high frequency FET cannot perform well.
FET have small gain bandwidth.

Applications of FET

FET is widely used in many electronics devices some of the applications of FET are as mentioned below.

Used in FM devices for amplification of frequency.
Used in memory chips.
Used in operational amplifier as variable voltage resistors.
Used in TV and FM receivers for mixer operation.
Used in constructing mixer circuit .

Read More : What is PNP Transistor?

Difference between BJT and FET Transistors Operation

BJT and FET both have different operations . In BJT both minority and majority charge carriers are responsible for the flow of current .In BJT emitter base junction is forward biased and the collector base junction is reverse biased. In NPN transistor due to less barrier potential in base emitter junction the electron flow from emitter to base and in base only few electron combine with holes because base is lightly doped and have fewer holes and this flow of electron leads to base current .The remaining electron from base region flows to reverse biased collector junction and leads to collector current .according to KCL .

I_{E =}I_B+ I_C

I_E = Emitter Current , I_B= Base Current , I_C= Collector Current

Working of PNP is same as NPN but majority charge carriers are holes in PNP transistor .

BJT operates in three different regions based on the biasing of the junctions :

Active Region : In this region emitter base junction is forward biased and collector base junction is reverse biased . Transistor works as amplifier in this region.
Saturation Region : In this region both emitter base junction and collector base junction is in forward bias .Transistor work as switch in this region.
Cut-off Region : In this region both emitter – base junction and collector base junction is in reverse bias . Transistor is OFF in this region.

where as FET is unipolar device in this only majority charge carriers are responsible for flow of current . FET have drain , gate , source and channel . The majority charge carriers enters from source and passes from channel and leads to drain . Therefore the current direction is from drain to source . The voltage applied in gate terminal is responsible for the amount of current that flows through channel that’s why FET is called voltage controlled device.

Difference between BJT and FET Transistors

The following table shows the major differences between bipolar junction transistor and field effect transistor.

Parameter	BJT	FET
Full form	BJT stands for Bipolar Junction Transistor.	FET stands for Field Effect Transistor.
Definition	A type of transistor which uses two types of charge carries viz. electrons and holes for conduction is known as bipolar junction transistor (BJT).	A type of transistor in which electric field is used to control the flow of current in a semiconductor is known as field effect transistor (FET).
Drive type	In BJT, the current flow is due to both majority and minority charge carriers. Thus, it is a bipolar device.	In FET, the electric current flows only due to majority charge carriers. Thus, it is a unipolar device.
Terminals	BJT has three terminals viz. Emitter, Base and Collector.	FET also has three terminals viz. Source, Drain and Gate.
PN junction	BJT consists of two PN junctions viz. emitter-base junction and collector-base junction.	FET does not have PN junctions.
Control element	BJT is a current-controlled device.	FET is a voltage controlled device.
Types	BJT are of two types: NPN transistor and PNP transistor.	FET are also of two types: N-channel FET and P-channel FET.
Configuration	BJT has three configurations: common emitter (CE), common base (CB) and common collector (CC).	FET also has three configurations: common source (CS), common gate (CG) and common drain (CD).
Size	BJT is large in size and hence requires more space. Therefore, it is more complicated to fabricate as an IC	FET is comparatively smaller in size. Hence, it is easier to fabricate as an IC.
Sensitivity	BJT is more sensitive to the changes in the applied voltage.	FET is less sensitive to the variations in the applied voltage.
Relationship between input and output	BJT has linear relationship between input and output.	FET has non-linear relationship between input and output.
Thermal noise	BJT has more thermal noise.	The thermal noise in case of FET is much lower.
Thermal runway	Thermal runway exits in BJT.	Thermal runway does not exists in FET.
Thermal stability	BJT has less thermal stability.	FET has good thermal stability due to absence of minority charge carriers.
Input impedance	In case of BJT, the input circuit is forward biased. Thus, the BJT has low input impedance.	FET has high input impedance due to reverse bias of input circuit.
Temperature coefficient at high current levels	BJT has positive temperature coefficient.	FET has negative temperature coefficient.
Suitability	BJT is suitable for low current applications.	FET is suitable for high current applications.
Switching speed	The switching speed of BJT is low.	FET has higher switching speed.
Effect of radiation	BJT is susceptible to radiation.	FET is relatively immune to radiation.
Gain bandwidth product	BJT has higher gain bandwidth product.	FET has lower gain bandwidth product.
Minority carrier storage effect	BJT suffers from minority carrier storage effect.	FET does not suffer from minority carrier storage effect.
Cost	BJT is cheaper to manufacture.	FET is relatively expensive to manufacture.
Installation	BJT does not require special handling during installation.	FET demands special handling during installation.
Applications	BJT is used as switch (in saturation and cut-off region) and amplifier (in active region).	FET is used as switch (in Ohmic and cut-off region) and as amplifier (in saturation region).

BJT and FET Transistors : Which One Should You Choose?

The choice between BJTs and FETs depends on the application’s specific requirements:

Choose BJT if you need high current amplification in analog circuits, such as audio equipment or signal amplification systems.
Choose FET if you need efficient switching, fast operation, or low-power consumption in digital circuits, power management, or portable devices.

How to Test BJT and FET Transistors Performance

Testing transistors involves measuring key parameters like gain, switching speed, and thermal stability. Here are some common methods:

For BJT:
- Use a multimeter to measure forward voltage drop and gain (hFE).
- Apply a small base current and observe the amplified output current.
For FET:
- Test gate-to-source voltage (Vgs) to observe conduction.
- Measure drain current (Id) for varying gate voltages.

Properties and Characteristics of BJT and FET Transistors

The following different properties differentiate both FET and BJT having different characteristics and applications.

Construction

BJT has a very simple and easier construction made from alternating semiconductor layers.
Either P-layer or N-layer is sandwiched between two N-layer or P-layers respectively.
FET has a little complex construction.
FET has either an N-channel or a P-channel between the gate of the P-layer or the N-layer respectively.
The channel is used for the flow of majority charge carriers.

PN Junctions

BJT transistor has two PN junctions between its collector and emitter.
One PN junction between Collector and Base and the other between Base and Emitter.
The FET transistor has no PN junction between its drain and source.

Charge Carrier

The BJT utilize both type of charge carriers for the flow of current.
During its operation, holes and electrons flow to conduct current.
The FET utilize only one type of charge carrier for the current flow.
It either uses holes in P-channel FET or electrons in N-channel FET.

Types

BJT has two types i.e. PNP and NPN
FET has two major types i.e. JFET (Junction FET) and MOSFET (Metal Oxide Semiconductor FET)
Each type of FET is further classified on the basis of channel i.e. N-channel and P-channel.

Terminals

The 3 terminals of BJT are named Collector, Base and Emitter.
Emitter and Collector are made of the same material with emitter having high doping rate.
The 3 terminals of FET are named Drain, Gate and Source.
The drain and source are the two ends of the channel made from the same type.

Input and Output

The BJT is a current controlled device.
Its input (at the base) is current which controls the output collector current.
The FET is a voltage-controlled device.
Its input is a voltage or potential difference (at the gate) which controls the output current at the source.

I/O Impedance

BJT operates by having a base-emitter (input) junction in forward bias.
Therefore, their input impedance is low.
Their output impedance is very high.
FET operates by having a gate at reverse bias.
Therefore, its input impedance is very high.
While its output impedance is very low.

Isolation

In BJT, the input at the base terminal is not isolated from the output.
In FET, the input at the gate terminal is in reverse bias and the input is isolated from the output.

Biasing

In BJT, the B-E junction is in forward bias while the C-B junction is in reverse bias.
IN FET, Gate is in reverse bias while the drain is at more positive voltage than the collector.

Gain

The BJT has very high gain due to its very high output impedance.
The FET has comparatively lower gain due to its low output impedance.

Terminal Swapping

In BJT, the terminals cannot be interchanged or swapped.
The emitter and collector are completely different terminals.
In FET, the drain and source terminal can be swapped.
The drain will be the terminal having more positive voltage.

Energy Consumption

The BJT takes current at its base terminal during continuous operation.
Therefore, it consumes energy and drains the battery.
The FET operates on the basis of the gate voltage.
Therefore, it is energy efficient and does not drain the battery.

Switching Speed

Since BJT use the flow of both type of charge carriers, its recovery time is slow.
Therefore, its switching speed is slow.
FET uses only one type of charge carrier having a fast recovery time.
Therefore, FET has a very fast switching speed.

Noise

The BJT is noisy and generates noise in the system. Therefore, not suitable for sensitive digital systems.
The FET is quite noiseless and is perfect for the sensitive system.

Cost

BJT having very simple construction is very cheap to manufacture.
FET having complex construction is comparatively costlier.

Size

The size of BJT is large. Therefore circuit made of BJT is bulkier.
The FET is more compact and smaller in size. Suitable for compact and small circuits.

FAQs on Difference between BJT and FET Transistors

JFET can operate in how many modes ?

JFET can operate in depletion mode .

In how many regions MOSFET operates ?

MOSFET operates in three regions that are Cut off Region , Triode Region , Saturation Region .

In how many modes MOSFET works ?

In two modes : Depletion and enhancement modes

Why BJT is called current control device ?

BJT is called current control device because input current is responsible for the amount of output current.

How many type of MOSFET are there ?

There are two types of MOSFET NMOS and PMOS

Difference between BJT and FET Transistors

What Is a BJT?

Advantages of BJT

Disadvantage of BJT

Applications of BJT

What Is a FET?

Advantage of FET

Disadvantage of FET

Applications of FET

Difference between BJT and FET Transistors Operation

Difference between BJT and FET Transistors

BJT and FET Transistors : Which One Should You Choose?

How to Test BJT and FET Transistors Performance

Properties and Characteristics of BJT and FET Transistors

Construction

PN Junctions

Charge Carrier

Types

Terminals

Input and Output

I/O Impedance

Isolation

Biasing

Gain

Terminal Swapping

Energy Consumption

Switching Speed

Noise

Cost

Size

FAQs on Difference between BJT and FET Transistors

Related Topics

Newsletter