BJT vs MOSFET | Difference Between BJT and MOSFET

BJT vs MOSFET is interesting material to learn before you decide to use which one is better.

Transistors are a type of semiconductor. BJTs and MOSFETs are electronic semiconductor devices that provide a big changing electrical output signal in exchange for minor input signal variations.

These transistors can be utilized as a switch or an amplifier because of this property. The first transistor was introduced in 1950, and it is considered one of the most important inventions of the twentieth century.

The technology is rapidly evolving, and several types of transistors have been introduced. BJT (Bipolar Junction Transistor) is the original type of transistor, and MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a later type of transistor.

This post explains the fundamental differences between BJT and MOSFET for a better grasp of the idea.

Both the BJT and MOSFET transistors are effective for amplification and switching. Despite this, they have vastly distinct traits.

What is BJT

The BJT (Bipolar Junction Transistor) is a semiconductor device that has mostly replaced vacuum tubes. The device is a current-controlled device, meaning that the collector or emitter output is a function of the base current.

A BJT transistor’s mode of operation is essentially dictated by the current at the base. The Emitter, Collector, and Base are the three terminals of a BJT transistor.

bjt vs mosfet 1

A BJT is actually a three-regioned piece of silicon. The P and N are two crossroads in which each region is given a separate name.

The NPN transistor and the PNP transistor are the two types of BJTs.

The charge carriers of the two types differ; NPN has holes as its principal carrier, whereas PNP contains electrons.

The working principles of the two types of BJT transistors, PNP and NPN, are nearly identical; the only differences are in biasing and power supply polarity.

Many people choose BJTs for low-current applications, such as switching, simply because they are less expensive.

BJT : How Does it Work

The functioning idea of a BJT is to manage the flow of current via the collector terminal by using voltage between the two terminals, such as base and emitter.

The configuration of a common emitter, for example, is seen in the diagram below.

bjt vs mosfet 2

The current entering a Base terminal is affected by the change in voltage, and this current, in turn, impacts the output current called.

This demonstrates that the input current regulates the output current flow.

As a result, this transistor is a current-controlled circuit.

What is MOSFET

The MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) or MOS transistor is a voltage-controlled device. There is no base current in this transistor, unlike the BJT.

A voltage on the gate, on the other hand, produces a field. This allows current to flow from the source to the drain. The voltage on the gate can either choke off or open this current flow.

A voltage on the oxide-insulated gate electrode can provide a channel for conduction between the two contacts “source” and “drain” in this transistor.

MOSFETs have the advantage of being more efficient in handling power.

MOSFETs have largely replaced the once-dominant BJTs as the most often utilized transistor in digital and analog circuitry.

bjt vs mosfet 3

These transistors come in four types:

  • P-channel
  • N-channel
  • Enhancement mode
  • Depletion mode

bjt vs mosfet 4

For N-channel MOSFETs, as well as P-channel MOSFETs, the source and drain terminals are built of N-type semiconductor. A metal oxide is used to separate the gate terminal from the source and drain terminals.

This insulation is a plus with this transistor because it results in low power consumption. As a result, p and n channel MOSFETs are employed as building blocks in digital CMOS circuitry to reduce power consumption.

Enhancement mode and depletion mode MOSFETs are the two types of MOSFETs available.

Depletion Mode: The channel shows its maximum conductance when the voltage on the ‘G’-terminal is low. Channel conductivity is reduced when the voltage on the ‘G’-terminal is positive or negative.

Enhancement Mode: The device does not conduct when the voltage on the ‘G’-terminal is low. The conductivity of this device improves as greater voltage is given to the gate terminal.

Read also : zener diode voltage regulator

MOSFET : How Does it Work

The MOS (metal oxide semiconductor), which is an essential component of the MOSFET, is responsible for its operation. The oxide layer is present between the source and drain terminals.

We can switch from p-type to n-type by applying +Ve or –Ve gate voltages.

When a +Ve voltage is given to the gate terminal, the holes under the oxide layer are forced down through the substrate by a repulsive force.

The bound –Ve charges linked with the acceptor atoms occupy this deflection zone.

bjt vs mosfet 5

Common Difference Between BJT vs MOSFET

In the list below, the differences between BJT vs MOSFET are discussed. As a result, the similarities between BJT vs MOSFET will be examined further down.

bjt vs mosfet 6


BJT : PNP or NPN, bipolar transistor

MOSFET : N-type or P-type, unipolar transistor


BJT : base, emitter, collector

MOSFET : gate, source, drain


BJT : current-controlled

MOSFET : voltage-controlled

Temperature coefficient

BJT : negative

MOSFET : positive


BJT : less expensive

MOSFET : more expensive

Electrostatic discharge

BJT : can not cause any issue

MOSFET : can cause an issue and problem


BJT : low current gain

MOSFET : high current gain


BJT : unstable, gain can be decreased by the increased collector current, gain can be increased by the increased temperature

MOSFET : stable for changing drain current

Input resistance

BJT : low

MOSFET : high

Input current

BJT : microamps to milliamps

MOSFET : picoamps

Input impedance

BJT : low

MOSFET : high

Switching speed

BJT : slower

MOSFET : faster

Switching frequency

BJT : low

MOSFET : high

Frequency response

BJT : inferior

MOSFET : superior

Potential drop

BJT : VCE is dropped about 200mV when saturated

MOSFET : VSD is dropped about 20mV when saturated


BJT : low current application

MOSFET : high current application

BJT vs MOSFET: What’s the Difference?

Below are the fundamental differences between BJT vs MOSFET transistors.

  • A bipolar junction transistor is known as a BJT, while a metal oxide semiconductor field-effect transistor is known as a MOSFET.
  • Base, emitter, and collector are the three terminals of a BJT, whereas source, drain, and gate are the three terminals of a MOSFET.
  • MOSFETs are utilized for high power applications, while BJTs are used for low current applications.
  • MOSFETs are utilized more frequently than BJTs in analog and digital circuits nowadays.
  • The current at the base terminal determines the operation of a BJT, while the voltage at the oxide insulated gate electrode determines the operation of a MOSFET.
  • A current-controlled device is a BJT, while a voltage-controlled device is a MOSFET.
  • In most situations, MOSFETs are preferred over BJTs.
  • MOSFETs have a more complicated construction than BJTs.

BJT vs MOSFET : Which one is a Better Amplifier?

Both the BJT and the MOSFET have their own set of advantages and downsides. However, because the matter is exceedingly subjective, we cannot determine which is better in BJT vs MOSFET.

However, there are various things to consider before choosing a BJT or MOSFET, such as the amount of power, efficiency, drive voltage, pricing, switching speed, and so on.

A MOSFET is typically utilized in power supply because its operation is faster than that of a BJT due to the use of metal oxide. BJT is dependent on the electron-hole pairing in this case.

Because MOSFETs have a fast switching speed, they work with minimal power once switched at high frequency, and they lead through the grid-oxide controlled field-effect rather than the recombination of electrons or holes like BJTs.

The gate control circuit of MOSFETs is fairly basic.

There are a number of causes behind this.

Conduction Losses are reduced

A bipolar junction transistor has a constant saturation voltage drop of 0.7 V, whereas a MOSFET has a 0.001-ohm on-resistance, which means it loses less power.

Higher Input Impedance

For operating a larger collector current, a bipolar junction transistor utilizes a low base current. They also work as a current amplifier.

The MOSFET is a voltage-controlled device that almost does not have any gate current.

Because the gain of power BJTs is medium to low, high base currents are required to produce high currents, the gate acts as a value capacitor, which is a substantial benefit in switching and high current applications.

When compared to BJT, the MOSFET takes up about a fifth of the space. The operation of a BJT is more complicated than that of a MOSFET.

As a result, FETs are simple to construct and can be utilized as passive elements instead of amplifiers.


There are numerous advantages to employing MOSFETs rather than BJTs, including the following.

Because the majority of charge carriers in a MOSFET carry current, it is far more sensitive than a BJT.

As a result, when compared to BJT, this device activates relatively quickly. As a result, this is mostly used to switch the power of SMPS.

The collector current of a MOSFET does not change much, whereas the collector current of a BJT changes due to temperature fluctuations, the transmitter’s base voltage, and current gain.

However, because MOSFET is a majority charge carrier, this significant change is not observed.

MOSFET input impedance is quite high, in the megohms area, whereas BJT input impedance is in the kiloohms region.

As a result, MOSFET fabrication is ideal for amplifier circuits.

MOSFETs have lower noise levels than BJTs. In this case, noise is defined as a signal’s random intrusion. When a transistor is used to boost a signal, the internal process of the transistor triggers some of this incidental interference.

When compared to MOSFETs, BJTs typically contribute a lot of noise into the signal.

As a result, MOSFETs are appropriate for signal processing in the absence of voltage amplifiers.

When compared to BJTs, the MOSFET is extremely tiny. As a result, they can be arranged in a smaller space.

MOSFETs are employed in computer and chip processors for this reason. In comparison to BJTs, MOSFETs have a very basic design.

BJT vs MOSFET : Temperature Coefficient

The temperature coefficient of a MOSFET is positive for resistance, making parallel operation of MOSFETs very straightforward.

When a MOSFET transmits amplified current, it quickly heats up, increases its resistance, and allows the current to flow to other devices in parallel.

Because the temperature coefficient of the bipolar junction transistor is negative, resistors are required throughout the parallel process.

Because the temperature coefficient of the MOSFET is positive, secondary breakdown does not occur.

Bipolar junction transistors, on the other hand, have a negative temperature coefficient, resulting in a secondary breakdown.

BJT : Advantages and Disadvantages

The following are some of the advantages of BJTs over MOSFETs.

  • When compared to MOSFETs, BJTs perform better in high-load circumstances and at higher frequencies.
  • When compared to MOSFETs, BJTs have superior fidelity and gain in linear areas.
  • Because of the low capacitance on the control pin, BJTs are much faster than MOSFETS. However, MOSFETs are more heat resistant and can be used to emulate a decent resistance.
  • BJTs are ideal for low-voltage and low-power applications.

The following are some of BJT’s disadvantages.

  • Radiation has an effect on it.
  • It makes more noise.
  • It’s less thermally stable.
  • BJT’s base control is quite complicated.
  • The switching frequency is modest, and the regulation is difficult.
  • When compared to voltage and current with a high alternating frequency, the switching time of a BJT is short.

MOSFET : Advantages and Disadvantages

The following are some of the advantages of MOSFETs.

  • A smaller size
  • Manufacturing is straightforward.
  • When compared to JFETs, the input impedance is large.
  • It is suitable for high-speed operation.
  • Because the power consumption is low, each chip can include more components outside of the region.
  • In digital circuits, the MOSFET with enhancement type is employed.
  • Because it lacks a gate diode, it can operate with a positive rather than a negative gate voltage.
  • When compared to JFET, it is more widely used.
  • MOSFET drain resistance is high due to low channel resistance.

The following are some of MOSFET’s disadvantages.

  • MOSFETs have a short lifespan.
  • For exact dose measurement, frequent calibration is required.
  • They are particularly susceptible to overload voltage, necessitating specific handling due to installation.

BJT vs MOSFET Summary

  1. A BJT is a bipolar junction transistor, whereas a MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor.
  2. A BJT has three elements: an emitter, collector, and base, whereas a MOSFET has three elements: a gate, source, and drain.
  3. BJTs are used in low-current applications, whereas MOSFETs are used in high-power applications.
  4. MOSFETs are now believed to be more widespread than BJTs in digital and analog circuits.
  5. The voltage at the oxide-insulated gate electrode determines the operation of a MOSFET, whereas the current at the base determines the operation of a BJT.

In most cases, a MOSFET is more efficient in power supplies. Using a BJT in a battery-powered device with a variable load and restricted power supply, for example, would be a horrible choice.

If a BJT is used to power anything with a predictable current draw (such as LEDs), this is good because the base-emitter current may be tuned to a fraction of the LED current for increased efficiency.

Thus, this is all about the distinctions between BJT and MOSFET, including what they are, how they function, the several types of MOSFET, and how they differ.

We hope you’ve gained a better understanding of this idea as a result of this discussion.

Furthermore, if you have any questions or concerns about this concept or electrical and electronics projects, please leave a comment in the box below.

Leave a Comment