Comparison between BJT, JFET and MOSFET in terms of mode of operation

B.J.T (Bipolar Junction transistor):

A transistor is a semiconductor device which has three operating regions and have separate functionality in each of those regions. The operating regions are as follows:

1. Active region-Amplify-Current controlled resistor.

Using these functionality analog modules like amplifiers can be designed

2. Saturation region-ON switch-Electrically controlled switch  

3. Cutoff region-Off Switch-Electrically controlled switch

Using these 2 and 3 functionalities, digital switching modules can be designed 

Here in B.J.T main current carrying path has both p and n junctions. So both electrons and hole charge carriers are used to perform the above functionalities, so this type of transistor is called bipolar junction transistor.

J.F.E.T (Junction gate Field Effect Transistor):

This three terminal transistor also has three operating regions and separate functionality in each of those regions.

1. Saturation region-Amplify-Voltage controlled device

In this region it doesn’t need biasing current to operate as amplifier. So input resistance is more and it can be used in small signal applications.

2. Linear region-ON switch-Electrically controlled switch 

3. Cutoff region-Off Switch-Electrically controlled switch

Here in J.F.E.T main current carrying path has no junctions, it has only one type of material either p-type or n-type. It is operated in depletion mode.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor):

When compared to JFET, MOSFET operating regions are same in depletion mode and it can be operated in enhancement mode also with very less gate leakage. So MOSFET is used in VLSI circuits and in small signal modules too.