What are the Differences between BJT and MOSFET?

The Transistors BJT & MOSFET are electronic semiconductor devices that give a large changing electrical o/p signal for small variations in small i/p signals. Due to this feature, these transistors are used as either a switch or an amplifier. The first transistor was released in the year 1950 and it can be treated as one of the most essential inventions in the 20th century. It is quickly developing the device and also various kinds of transistors have been introduced. The first type of transistor is BJT (Bipolar Junction Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is another type of transistor introduced later. For a better understanding of this concept, here this article gives the main difference between BJT and MOSFET.

What is BJT?

A bipolar junction transistor is one type of semiconductor device and in the olden days, these devices are used in the place of vacuum tubes. The BJT is a current-controlled device where the o/p of the base terminal or emitter terminal is a function of the current in the base terminal. Fundamentally, the operation of a BJT transistor is determined by the current at the base terminal. This transistor consists of three terminals namely the emitter, base, and collector. Actually, a BJT is a silicon piece that includes three regions and two junctions. The two regions are named the P-junction and N-junction.

Bipolar Junction Transistor
Bipolar Junction Transistor

There are two kinds of transistors namely PNP and NPN. The main difference between BJT and MOSFET is their charge carriers. In PNP transistor, P stands for positive and the majority charge carriers are holes whereas in NPN transistor, N stands for negative and the majority charge carriers are electrons. The operating principles of these transistors are practically equal and the main difference is in biasing as well as the polarity of the power supply for each type. BJTs are apt for low current applications like switching purposes.

Working Principle of BJT

The working principle of a BJT involved in the use of Voltage between the two terminals such as base and emitter to regulate the flow of current through collector terminal. For instance, the configuration of a common emitter is shown in the figure below.

Working Principle of BJT
Working Principle of BJT

The change in voltage affects the current entering in a Base terminal and this current will, in turn, affect the o/p current called. By this, it is shown that the input current controls the flow of o/p current. So this transistor is a current controlled device. Please follow the below link to know more about; the Major Difference between BJT and FET.

What is MOSFET

The MOSFET is one kind of FET (Field Effect Transistor), which consists of three terminals namely gate, source, and drain. Here, the drain current is controlled by the voltage of the gate terminal Therefore, these transistors are voltage-controlled devices.


Metal Oxide Semiconductor Field Effect Transistor
Metal Oxide Semiconductor Field Effect Transistor

These transistors are available in 4 different types such as P-channel or N-channel with either in enhancement mode or depletion mode. The source and Drain terminals are made of N-type semiconductor for N-channel MOSFETs and equally for P-channel devices. The gate terminal is made of metal and detached from source & drain terminals using a metal oxide. This insulation roots low power consumption & it is a benefit in this transistor. Therefore, this transistor is used where p and n channel MOSFETs are used as building blocks to reduce the power consumption like digital CMOS logic.

MOSFETs are classified into two types such as enhancement mode and depletion mode

Depletion Mode: When the voltage on the ‘G’-terminal is low, then the channel shows its max conductance. As the voltage on the ‘G’-terminal is positive or negative, then channel conductivity will be decreased.

Enhancement Mode: When the voltage on the ‘G’-terminal is low, then the device does not conduct. When the more voltage is applied to the gate terminal, then the conductivity of this device is good.

Please follow the below link to know more about; What is MOSFET with Working?

Working Principle of MOSFET

The working of MOSFET depends upon the MOS (metal oxide capacitor) which is the essential part of the MOSFET. The oxide layer presents, among the two terminals such as source and drain. By applying +Ve or –Ve gate voltages, we can set from p-type to n-type. When +Ve voltage is applied to the gate terminal, then the holes existing under the oxide layer with a repulsive force and holes are pushed down through the substrate. The deflection region occupied by the bound –Ve charges which are associated with the acceptor atoms.

Working Principle of MOSFET
Working Principle of MOSFET

Difference between BJT and MOSFET

The main difference between BJT and MOSFET transistors are discussed below.

Difference between BJT and MOSFET
Difference between BJT and MOSFET
  • The BJT is a bipolar junction transistor whereas MOSFET is a metal oxide semiconductor field-effect transistor.
  • A BJT has three terminals namely base, emitter, and collector, while a MOSFET has three terminals namely source, drain, and gate.
  • BJT’s are used for low current applications, whereas MOSFET is used for high power applications.
  • Nowadays, in analog and digital circuits, MOSFETs are treated to be more commonly used than BJTS.
  • The working of BJT depends on the current at the base terminal and the working of the MOSFET depends on the voltage at the oxide insulated gate electrode.
  • The BJT is a current controlled device and MOSFET is a voltage-controlled device.
    MOSFETs are used more than BJTs in most of the applications
  • The structure of the MOSFET is more complex than BJT

Thus, this is all about the difference between BJT and MOSFET which includes what are BJT and MOSFET, working principles, types of MOSFET, and differences. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or electrical and electronics projects, please give your feedback by commenting in the comment section below. Here is a question for you, what are the BJT and MOSFET characteristics?


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