14 Difference Between BJT And FET (With Comparison Chart)

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What Is BJT (Bipolar Junction Transistor)?

A bipolar junction transistor (BJT) is a three terminal electronic device that amplifies the flow of current. The bipolar junction transistors are formed by sandwiching either n-type or p-type semiconductor layer between pairs of opposite polarity semiconductor layers. In bipolar junction transistor (BJT), electric current is conducted by both free electrons and holes.  Unlike a normal PN-junction diode, the transistor has two p-n junctions.

BJT consist of three terminals that is, emitter, base and collector which are denoted by E, B and C respectively. The emitter (E) section is heavily doped so that it can inject a large number of charge carriers into the base. The Base (B) section is lightly doped and is very thin as compared to emitter and collector. On the other hand, collector (C) section is moderately doped, its main function is to collect charge carriers.

Bipolar junction transistors are mainly used in switching and amplification. The various applications of bipolar junction transistors include:

  • Radio transmitters
  • Audio amplifiers
  • Televisions
  • Computers
  • Mobile Phones

Types Of Bipolar Junction Transistor (BJT)

Bipolar junction transistors are classified into two types based on their construction. They include:

  • NPN Transistor– In this transistor, a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers.
  • PNP Transistor – In this transistor, a single n-type semiconductor layer is sandwiched between two p-type semiconductor layers.

What You Need To Know About Bipolar Junction Transistor (BJT)

  1. Bipolar Junction Transistors are bipolar devices in which there is a flow of both majority and minority charge carriers.  
  2. BJT consist of three terminals that is, emitter, base and collector which are denoted by E, B and C respectively.
  3. BJTs are current controlled. They require a bias current to the base terminal to operate.
  4. The input circuit of BJT is forward biased and therefore BJT has low input impedance. What this means is that they draw more power into the power circuit, which lead to load on the circuit.
  5. In BJT, the base current controls the output current.
  6. BJT has more noisy operation.
  7. BJT has a positive temperature coefficient at high current levels. It leads to thermal breakdown.
  8. BJT suffers from minority carrier storage effects and therefore it has lower switching speed and cut off frequencies.
  9. BJT are larger in size and therefore require more space than FETs normally.
  10. BJT are most preferred during the application of low current.
  11. In BJT the relation between input and output is considered to be linear.
  12. BJTs are cheaper to produce.
  13. Due to reduction in minority carrier lifetime, the performance of BJT is degraded by neutron radiation.
  14. BJTs are less popular and less frequently used.

What Is FET (Field Effect Transistor)?

The Field-Effect Transistor (FET) is a type of transistor which uses an electric field to control the flow of current. They are commonly referred to as unipolar transistors because they involve single-carrier-type operation that is, they use electrons or holes as charge carriers in their operation, but not both.  FET is commonly used for weak-signal amplification for example amplifying wireless signals. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator.

Also, Field Effect Transistors can be made much smaller than an equivalent BJT transistor and along with their low power consumption and power dissipation makes them ideal for use in integrated circuits such as the CMOS range of digital logic chips.

FET is a three terminal device that is constructed with no PN-junctions within the main current carrying path between the Drain and the source terminals.  These terminals -Source, Drain and Gate which are denoted by S, D and G respectively corresponds in function to the Emitter, Base and Collector of the bipolar transistor. The current path between these two terminals is referred to as the ‘’channel’’ which may be made of either a P-type or an N-type semiconductor material.

There are two main types of field effect transistor, that is:

  • The Junction Field Effect Transistor (JFET).
  • The Insulated-Gate Field Effect Transistor (IGFET), which is more commonly referred to as the standard Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

What You Need To Know About Field Effect Transistor (FET)

  1. Field Effect Transistors are unipolar devices, in which only the majority carriers flows.
  2. FET consist of three terminals that is, Source, drain and gate which are denoted by S, D and G respectively.
  3. FETs are voltage controlled. They only require a voltage applied to the door to enable or disable the FET.
  4. The input circuit of FET is reversed biased and therefore FET exhibits relatively higher impedance. What this means is that they consume virtually no energy and therefore do not overload the circuit that feeds them.
  5. In FET, the Gate voltage controls the output current.
  6. FET is less noisy compared to BJT. It is more suitable for input stages of low level amplifiers.
  7. FET has a negative temperature coefficient at high current levels. It prevents the FET from thermal breakdown problem.
  8. FET does not suffer from minority carrier storage effects and therefore it has higher switching speed and cut off frequencies.
  9. FETs are relatively smaller in size especially in case of integrated circuits composed of many transistors.
  10. FETs are most preferred during the applications of high current.
  11. In FET, the relation between input and output is considered to be non-linear.
  12. FETs are relatively costly to produce.
  13. FET operation does not depend upon the minority carriers and therefore they can tolerate much higher level of radiation.
  14. FETs are more popular around the world and most of the current application or devices use FETs.

Also Read: Difference Between JFET And MOSFET

Difference Between BJT And FET In Tabular Form

BASIS OF COMPARISON BJT FET
Description Bipolar Junction Transistors are bipolar devices in which there is a flow of both majority and minority charge carriers.    Field Effect Transistors are unipolar devices, in which only the majority carriers flows.  
Terminals It consist of three terminals that is, emitter, base and collector which are denoted by E, B and C respectively.   It consist of three terminals that is, Source, drain and gate which are denoted by S, D and G respectively.  
Functionality BJTs are current controlled. They require a bias current to the base terminal to operate.   FETs are voltage controlled. They only require a voltage applied to the door to enable or disable the FET.  
Impedance The input circuit of BJT is forward biased and therefore BJT has low input impedance. The input circuit of FET is reversed biased and therefore FET exhibits relatively higher impedance.
Output Current Control The base current controls the output current.   The Gate voltage controls the output current.  
Noise BJT has more noisy operation.   FET is less noisy compared to BJT.
Temperature Coefficient BJT has a positive temperature coefficient at high current levels. FET has a negative temperature coefficient at high current levels.
Switching Speed & Cut off Frequencies It has lower switching speed and cut off frequencies.   It has higher switching speed and cut off frequencies.  
Size BJT are larger in size and therefore require more space than FETs normally.   FETs are relatively smaller in size especially in case of integrated circuits composed of many transistors.    
Suitability BJT are most preferred during the application of low current.   FETs are most preferred during the applications of high current.  
Relationship Between Input And Output In BJT the relation between input and output is considered to be linear.   In FET, the relation between input and output is considered to be non-linear.  
Cost BJTs are cheaper to produce.   FETs are relatively costly to produce.  
Effect Of Radiation Due to reduction in minority carrier lifetime, the performance of BJT is degraded by neutron radiation.   FET operation does not depend upon the minority carriers and therefore they can tolerate much higher level of radiation.  
Usage BJTs are less popular and less frequently used.   FETs are more popular around the world and most of the current application or devices use FETs.  

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