Difference between FET and BJT

Field effect transistors:

FET is a majority carrier device,and is,therefore,often called a unipolar transistor while BJT operates by the injection and collection of minority carriers;since in the device, both holes and electrons take part in conduction it is called a bipolar transistor.Like its bipolar counterpart,the FET is a three-terminal (namely,drain,source and gate) semicondctor device in which current conduction is by-one type of majority carriers(electrons in case of an N-channel FET or holes in a P-channel FET).unlike the BJT, FET needs virtually no input (bias signal) current.field-effect devices are controlled by an electric field(or voltage) and give an extremely high input impedance-most important advantage over BJT devies.

Field-effect transistors(FETs )are of several types such as junction field-effect transistors(JFETs),metal semiconductor field-effect transistors(MESFETs) and metal-insulator semiconductor field-effect transistors (MISFETs).

• FET is majority carrier device,and is,therefore,often called a unipolor transistor.
• FET is a three terminal device (Gate,source,drain).
• FET is a voltage controller device.
• FET give an extremely high input impedance.
• many types of FETs are present...JFETs,MESFETs and MISFETs.   

picture of a  chip (using FET)

Bipolar junction transistors:

A transistor may consist of a layer of P-type material sandwiched between two layers of N-type material as shown in fig in the former case the transistors is referred to as a P-N-P transistor and in latter case,as N-P-N transistor.Each type of transistor has two P-N junctions-one junction between the emitter and base,called the emitter-base junction or simply the emitter junction and the other junction between the base and collector junction.thus a transistor is like two P-N junction diodes have all the characteristics. The two junctions give rise to three region provided with three terminals called the emitter,base and collector as shown in fig.the emitter, base and collector correspond in a general way to the cathode,grid and plate or anode of a vacuum triode.

picture of BJT (three terminal shown in figure base,collect,emitter)



• BJT operates by the injection and collection of minority carriers.
• in BJT both holes and electrons take part in conduction,so it is called bipolar transistor.
• it is a current controller device.
• BJT is  a three terminal device (emitter,collector & base).
• BJT is referred to as a P-N-P transistor and N-P-N.

  

  N-P-N transistor