Difference Between BJT and FET

The transistor is a three-terminal semiconductor device, it is an active device that is used in most of the electronics devices. Transistors can be used as insulators and as conductors as needed. we can use a transistor as a switch and as an amplifier. It can be used with other circuit elements and can be able to amplify current and voltage. We can classify transistors into two types that is Bipolar, and Field Effect Transistor.

BJT is a three-terminal device, it has two PN junctions. It is mostly used analog electronic device because of its high current gain. FET is a very important component in today’s electronic devices. FET is a type of device in which electric fields are used to control the flow of charges or current flowing through a channel. Due to its high input impedance, low output impedance, and high gain capacity, they are widely used in electronic devices.

BJT and FET both are used in many different devices and they have many different characteristics that make them different from each other. In this article, we will go through all the things that make them different and we will get the overview of BJT and FET.

BJT vs FET

What is 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

• PNP BJT

NPN BJT

Symbol of NPN and PNP Transistor

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.

What is 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
  
  

  N-channel FET and P-channel FET

Symbol of N-channel FET and P-channel FET

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.

JFET and MOSFET Transistor

Comparison Between BJT and FET 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

In following table some of the differences of BJT and FET is mentioned

BJT	FET
BJT stands for Bipolar Junction Transistor.	FET stands for Field Effect Transistor.
It is current controlled device.	It is voltage controlled device.
In BJT both minority and majority charge carriers are responsible for flow of current.	In FET only majority charge carriers are responsible for flow of current.
It has three terminal Emitter , Base and collector.	It has Drain , Source , Gate and Body but mostly body terminal is sorted so it is also three terminal device.
BJT have low input impedance .	FET have high input impedance.
BJT is bipolar device.	FET is unipolar device.
BJT requires large chip area during fabrication .	FET requires small chip area during fabrication.
It is of two types NPN and PNP BJT.	It is also of two types N channel FET and P channel FET.
Switching speed is slower in BJT .	Switching speed is faster in FET.
BJT generate more noise.	FET generate less noise.
BJT is more susceptible to radiation.	FET is less susceptible to radiation.
Thermal stability is less is BJT.	FET is more thermal stable.
BJT consumes high power.	FET consumes less power.
Manufacturing coast is less .	High manufacturing coast.
BJT has higher gain bandwidth product.	FET has lower bandwidth product.
BJT has higher voltage drop .	FET has less voltage drop.
BJT is complex to fabricate.	FET is less complex to fabricate than BJT.
BJT is mainly used in low power applications.	FET is used in high power applications.
BJT has linear input – output relation.	FET has non linear input – output relation.
It has high voltage gain and less current gain.	It has low voltage gain and high current gain.

Advantages of BJT and FET

Following are the some of the advantages of BJT and FET :

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 .

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 BJT and FET

Following are some of the disadvantage of BJT and FET :

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.

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 BJT and FET

Following are the some of applications of BJT and FET :

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.

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 .

Conclusion

From the above article we got an overview of what is BJT and FET their operational difference and advantages and disadvantages along with their applications. Their are many difference between BJT and FET as mentioned above . Although BJT and FET are basic component for many electronics appliances . BJT and FET both are widely used in many devices , we can use them in any device based on the requirement of device. The main function of BJT and FET are for amplification and switching based on our needs they are used.

FAQs on Difference Between BJT and FET

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