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