Know the Major Difference between BJT and FET

BJTs and FETs are two different kinds of transistors and also known as active semiconductor devices. The acronym of the BJT is Bipolar Junction Transistor and FET stands for Field Effect Transistor. BJTS and FETS are available in a variety of packages based on the operating frequency, current, voltage and power ratings. These types of devices allow a greater degree of control over their working. BJTS and FETs can be used as switches and amplifiers in electrical and electronics circuits. The major difference between BJT and FET is that, in a field effect transistor only majority charge carries flows, whereas in BJT both majority and minority charge carriers flows.

Difference Between BJT and FET

The main difference between BJT and FET is discussed below, which includes what is BJT and FET, construction and working of BJT and FET.

Difference between BJT and FET
Difference between BJT and FET

What is BJT?

The BJT is a one type of transistor that uses both majority and minority charge carriers. These semiconductor devices are available in two types such as PNP and NPN. The main function of this transistor is to amplify current. These transistors can be used as switches and amplifiers. The applications of BJTs involve in a wide range that includes electronic devices like TVs, mobiles, computers, radio transmitters, audio amplifiers and industrial control.

Bipolar Junction Transistor
Bipolar Junction Transistor

Construction of BJT

A bipolar junction transistor comprises of two p-n junctions. Depending on the structure of the BJT, these are classified into two types such as PNP and NPN. In NPN transistor, lightly doped P-type semiconductor is placed between two heavily-doped N-type semiconductors. Equally, a PNP transistor is formed by placing an N-type semiconductor between P-type semiconductors. The construction of a BJT is shown below. The emitter and collector terminals in the below structure are called n-type and p-type semiconductors which are denoted with ‘E’ and ‘C’. While the remaining collector terminal is called p-type semiconductor denoted with ‘B’.

Construction of BJT
Construction of BJT

When a high voltage is connected in reverse bias mode across both the base and collector terminals. This roots a high depletion region to form across the BE junction, with a strong electric field that stops the holes from the B-terminal to the C-terminal. Whenever the E and the B terminals are connected in forward bias, the flow of electrons direction will be from the emitter terminal to the base terminal. In the base terminal, some electrons recombine with the holes, but the electric field across the B-C junction attracts electrons. Most electrons end up overflowing into the collector terminal to create a huge current.

Since the flow of heavy current through the collector terminal can be controlled by the small current through the emitter terminal. If the potential difference across the BE junction is not strong, then the electrons are not able to get into the collector terminal so, there is no flow of current through the collector terminal. Due to this reason, a bipolar junction transistor is used as a switch as well. The PNP junction also works with a same principle, but the base terminal is made with a N-type material and the majority charge carriers in the PNP transistor are holes.

What is FET?

The term FET stands for Field effect transistor and it is also named as a Uni polar transistor. FET is a one type of transistor, where the o/p current is controlled by electric fields. The basic type of FET is totally dissimilar from BJT. FET consist of three terminals namely source, drain and gate terminals.The charge carriers of this transistor are holes or electrons, which flow from the source terminal to drain terminal via an active channel. This flow of charge carriers can be controlled by the voltage applied across the source and gate terminals.

Field Effect Transistor
Field Effect Transistor

Construction of FET

Field effect transistors are classified into two types such as JFET and MOSFET. These two transistors have similar principles. The construction of p-channel JFET is shown below. In p-channel JFET, the majority charge carriers flow from the source to drain. Source and drain terminals are denoted by S and D.

P-Channel JFET
P-Channel JFET

The gate terminal is connected in reverse bias mode to a voltage source so that a depletion layer can be form across the regions of the gate and the channel where charges flow. Whenever the reverse voltage on the gate terminal is increased, the depletion layer increases. So it can stop the flow of current from the source terminal to the drain terminal. So, by changing the voltage at the gate terminal, the flow of current from the source terminal to the drain terminal could be controlled.

Difference between BJT and FET

  • Bipolar junction transistors are bipolar devices, in this transistor there is a flow of both majority & minority charge carriers.
  • Field effect transistors are unipolar devices, in this transistor there are only the majority charge carriers flows.
  • Bipolar junction transistors are current controlled.
  • Field effect transistors are voltage controlled.
  • In many applications FETs are used than bipolar junction transistors.
  • Bipolar junction transistor consist of three terminals namely emitter, base and collector. These terminals are denoted by E, B and C.
  • Field effect transistor consist of three terminals namely source, drain and gate. These terminals are denoted by S, D and G.
  • The input impedance of field effect transistors has high compared with bipolar junction transistors.
  • A BJT needs a small amount of current to switch on the transistor.The heat dissipated on bipolar stops the total number of transistors that can be fabricated on the chip.
  • Whenever the ‘G’ terminal of the FET transistor has been charged, no more current is required to keep the transistor ON.
  • The BJT is responsible for overheating due to a negative temperature co-efficient.
  • FET has a +Ve temperature coefficient for stopping over heating.
  • BJTs are applicable for low current applications.
  • FETS are applicable for low voltage applications.
  • FETs have low to medium gain.
  • BJTs have a higher max frequency and a higher cutoff frequency.

Thus, this is all about comparison between BJT and FET, includes what is BJT and FET, Construction of BJT, construction of FET, differences between BJT and FET. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or electronics projects please comment in the comment section below. Here is a question for you, what are the applications of BJT and FET?

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14 Comments

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    1. Tarun Agarwal says:

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  3. Thank you. it’s useful to me

  4. I got better understanding in difference between bjt and get.
    Why bjt is most commonly used in feedback amplifiers than fet?

    1. Tarun Agarwal says:

      Hi Veera
      The BJT prefer for low current application for switching purpose, for instance, BJT is cheaper also
      Thank you so much for your feedback
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  5. bharath kumar says:

    thanks for keeping this information .it is very clear and clarity

    1. Tarun Agarwal says:

      Hi Bharath

      I sincerely appreciate your kind response regarding my article

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  6. Thank you for clearing my doubts

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