Difference between BJT and MOSFET

BJTs are three-terminal semiconductor devices used for switching and amplification of signals. MOSFETs, on the other hand, are four-terminal semiconductor devices used for switching applications. Read through this article to find out more about BJTs and MOSFETs and how they differ from each other.

What is BJT?

BJT stands for Bipolar Junction Transistor. It is a type of transistor in which the current flow is due to two types of charge carriers viz. electrons and holes. A BJT consists of three alternating layers of P-type and N-type semiconductor materials and two P-N junctions. The BJT has three terminals viz. emitter (E), base (B), and collector (C).

When P-type semiconductor material is sandwiched between two N-type layers, the bipolar junction transistor is known as NPN transistor. When N-type layer is sandwiched between two P-type layers, the resulting BJT is known as PNP transistor.

What is MOSFET?

MOSFET stands for metal oxide semiconductor field effect transistor. It is a four-terminal semiconductor device where the terminals are: source (S), drain (D), gate (G), and body (or substrate). MOSFET is a field effect transistor with a MOS structure. MOSFETs are high speed and low-loss operation transistors.

According to the principle of operation, the MOSFETs are of two types viz. −

Enhancement MOSFET
Depletion MOSFET

An enhancement type MOSFET remains OFF under normal conditions and needs a gate voltage to turn it ON. While the depletion MOSFET remains ON under normal conditions and it requires a gate voltage to turn it OFF.

Difference between BJT and MOSFET

The following table highlights the major differences between bipolar junction transistor and metal oxide semiconductor field effect transistor.

Parameter	BJT	MOSFET
Full form	BJT stands for Bipolar Junction Transistor.	MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
Definition	BJT is a three-terminal semiconductor device used for switching and amplification of signals.	MOSFET is a four-terminal semiconductor device which is used for switching applications.
Types	Based on the construction, BJTs are classified into two types: NPN and PNP.	Based on the construction and operation, the MOSFETs are classified into four types: P-channel enhancement MOSFET, N-channel enhancement MOSFET, P-channel depletion MOSFET and N-channel depletion MOSFET.
Terminals	BJT has three terminals viz. emitter, base and collector.	MOSFET has four terminals, i.e., source, drain, gate and body (or substrate).
Charge carriers	In BJT, both electrons and holes act as charge carriers.	In MOSFET, either electrons or holes act as charge carriers depending on the type of channel between source and drain.
Polarity	BJT is a bipolar device.	MOSFET is a unipolar device.
Controlling quantity	BJT is a current controlled device.	MOSFET is a voltage controlled device.
Input impedance	BJT has low input impedance.	MOSFET has relatively high input impedance.
Temperature coefficient	BJT has negative temperature coefficient.	MOSFET has positive temperature coefficient.
Switching frequency	The switching frequency BJT is low.	For MOSFET, the switching frequency is relatively high.
Power consumption	BJT consumes more power than MOSFET.	The power consumed by a MOSFET is less than BJT
Applications	BJT is preferred for the low current applications. It is widely used as amplifiers, oscillators and electronic switches.	MOSFET is suitable for high power applications. It is used in power supplies, etc.

Conclusion

BJT and MOSFET are types of semiconductor transistors with a wide range of applications. However, as we have highlighted in the above table, they are different from each other in many aspects. It is important to understand the way BJTs and MOSFETs function in order to use either of them as per the exact requirements.