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 of 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 the PNP transistor, P stands for positive and the majority charge carriers are holes whereas in the 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.

BJT Symbol
BJT Symbol

Working Principle of BJT

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

Bipolar Junction Transistor Working
Bipolar Junction Transistor Working

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.

PCBWay

MOSFET
MOSFET

These transistors are available in 4 different types such as P-channel or N-channel with either an 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 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.

MOSFET Block Diagram
MOSFET Block Diagram

Differences between BJT and MOSFET

The difference between BJT and MOSFET in tabular form is discussed below. So the similarities between BJT and MOSFET are discussed below.

Difference between BJT and MOSFET
Difference between BJT and MOSFET

BJT

MOSFET

BJT is PNP or NPNMOSFET is N-type or P-type
BJT is a current controlled deviceMOSFET is a voltage-controlled device
The temperature coefficient of BJT is negativeThe temperature coefficient of MOSFET is positive
The current output of the BJT can be controlled through the i/p base current.The current output of the MOSFET can be controlled through the i/p gate voltage.
BJT is not expensiveMOSFET is expensive
In BJT, Electrostatic Discharge is not a problem.In MOSFET, Electrostatic Discharge is an issue, so it can cause a problem.
It has low current gain & it is not stable. Once the collector current increases then the gain can be decreased. If the temperature increases then the gain can also be increased.It has a high current gain which is almost stable for changing drain currents.
The input resistance of BJT is low.The input resistance of MOSFET is high.
Input current is Milliamps/ MicroampsInput current is Picoamps
When the BJT is saturated then less heat dissipation can occur.When the MOSFET is saturated then less heat dissipation can occur.
The switching speed of the BJT is slowerThe switching speed of the MOSFET is higher
The frequency response is inferiorThe frequency response is better
Once it is saturated, then the potential drop across the Vce is about 200 mV.Once it is saturated, then the potential drop among the source and drain is about 20 mV.
The base current of the BJT starts to supply using an +0.7V of the input voltage. Transistors can be operated through large base currentsThe N-channel MOSFETs use +2v to +4v to switch ON them and the gate current of this is about zero.
The input impedance is lowThe input impedance is high
The switching frequency of BJT is lowThe switching frequency of MOSFET is high
It is used for the low current applicationIt is used for the high current application

Key Differences between BJT and MOSFET

The key differences between BJT and MOSFET transistors are discussed below.

  • 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

Which is Better Amplifier BJT or MOSFET?

Both the BJT and MOSFET include unique features and their own advantages and disadvantages. But, we cannot say which is good in BJT & MOSFET as the matter is extremely subjective. But before selecting the BJT or MOSFET, there are several factors that need to consider like the level of power, efficiency, drive voltage, price, speed of switching, etc

Usually, a MOSFET is used in power supplies more efficiently because the working of MOSFET is faster due to metal oxide usage apart from BJT. Here, BJT depends on the combination of electron-hole.
MOSFET works with low power once switching at high frequency because it has a quick switching speed so it leads through grid-oxide controlled field-effect but not through the recombination of an electron or hole like BJT. In MOSFET, the circuit like gate control is very simpler
There are numerous reasons that stand out

Fewer Conduction Losses

A bipolar junction transistor includes a stable saturation voltage drop like 0.7 V, whereas the MOSFET includes a 0.001-ohm on-resistance that leads to fewer power losses.

High Input Impedance

A bipolar junction transistor uses a low base current for operating a larger collector current. And they perform like a current amplifier. The MOSFET is a voltage-controlled device and it doesn’t include gate current almost. The gate works like a value capacitor and it is a significant benefit in the applications of switching & high current because the gain of the power BJTs has medium to low, that needs high base currents to produce high currents.

The area occupied by the MOSFET is less as compared with BJT like 1/5th. The BJT operation is not as simple as compared with MOSFET. So FET can be designed very easily and can be used like passive elements instead of amplifiers.

Why is MOSFET better than BJT?

There are many benefits of using MOSFET instead of BJT like the following.

MOSFET is very responsive as compared with BJT because the majority of charge carriers in the MOSFET are the current. So this device activates very quickly as compared with BJT. Thus, this is mainly used for switching the power of SMPS.

MOSFET does not undergo huge changes whereas, in BJT, the collector current of this will change because of the temperature changes, the transmitter’s base voltage, and current gain. However, this vast change is not found within MOSFET because it is a majority charge carrier.

The input impedance of MOSFET is very high like the megohms range whereas the BJT’s input impedance ranges within the kiloohms. Therefore, MOSFET making is extremely perfect for amplifier based circuits.

As compared with BJTs, MOSFETs have less noise. Here noise can be defined as the random intrusion within a signal. Once a transistor is utilized to increase a signal, then the transistor’s internal process will initiate some of this casual interference. Generally, BJTs introduce huge noise into the signal as compared with MOSFETs. So MOSFETs are suitable for processing the signal otherwise voltage amplifiers.

The size of the MOSFET is very small as compared with BJTs. So the arrangement of these can be done in less space. For this cause, MOSFETs are used within the processors of computer & chips. So, the design of MOSFETs is very simple as compared with BJTs.

Temperature Coefficient of BJT & FET

The temperature coefficient of MOSFET is positive for resistance and this will make MOSFET’s parallel operation very simple easy. Primarily, if a MOSFET transmits amplified current, very easily it heats up, increases its resistance, and causes this flow of current to move to other devices within parallel.

The temperature coefficient of BJT is negative, so resistors are essential throughout the parallel process of the bipolar junction transistor.

The secondary breakdown of MOSFET does not happen since the temperature coefficient of this is positive. However, bipolar junction transistors have a negative temperature coefficient so it results in a secondary breakdown.

Advantages of BJT over MOSFET

The advantages of BJT over MOSFET include the following.

  • BJTs operate better in high load conditions & with higher frequencies as compared with MOSFETS
  • BJTs have higher fidelity & better gain in the linear areas as evaluated with the MOSFETs.
  • As compared with MOSFETS, BJTS are very faster because of the low capacitance on the control pin. But MOSFET is more tolerant to heat & can simulate a good resistor.
  • BJTs are a very good choice for voltage and low power applications

The disadvantages of BJT include the following.

  • It affects by radiation
  • It generates more noise
  • It has less thermal stability
  • Base control of BJT is very complex
  • Switching frequency is low & high complex control
  • The switching time of BJT is low as compared with voltage & current with high alternating frequency.

Advantages and Disadvantages of MOSFET

The advantages of MOSFET include the following.

  • Less size
  • Manufacturing is simple
  • Input impedance is high as compared with JFET
  • It supports high-speed operation
  • Power consumption is low so that more components can be allowed for each chip outside the area
  • The MOSFET with enhancement type is used in digital circuitry
  • It doesn’t have a gate diode, so it is possible to work through a positive otherwise negative gate voltage
  • It is broadly used as compared with JFET
  • The drain resistance of MOSFET is high because of low channel resistance

The disadvantages of MOSFET include the following.

  • The disadvantages of MOSFET include the following.
  • The lifespan of MOSFET is low
  • Frequent calibration is required for precise dose measurement
  • They have extremely vulnerable to overload voltage; therefore special handling is to be necessary because of installation

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?

6 Comments

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