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Difference between BJT and FET
Both BJTs and FETs are transistors widely used in different types of electronic applications. Go through this article to find out the important features of BJTs and FETs and how they differ from each other in the way they function.
What is BJT?
BJT stands for Bipolar Junction Transistor. The BJT is the type of transistor in which the current flow is due to two types of charge carriers viz. electrons and holes. A BJT consists of three alternating layers of P-type and N-type semiconductor materials and two P-N junctions.
When P-type semiconductor material is sandwiched between two N-type layers, the bipolar junction transistor is known as NPN transistor. When N-type layer is sandwiched between two P-type layers, the resulting BJT is known as PNP transistor. The circuit symbols of NPN and PNP type bipolar junction transistors are shown in Figure-1.
The BJT has three terminals viz. Emitter, Base and Collector. Each of these terminals is connected to each layer of BJT. Here, the emitter and collector layers are heavily doped, the collector layer is doped lightly than the emitter. The middle layer is the base of BJT that is most lightly doped.
What is FET?
FET stands for Field Effect Transistor. The flow of current in the field effect transistors is due to only one type of charge carrier (either hole or electron). For this reason, they are known as unipolar transistors. The FET is also a three terminal device, where the names of the terminals are Source, Drain and . ThGatee circuit symbol of FET is shown in Figure-2.
Based on the channel between the source and drain provided for current to flow, the FET is of two types viz. N-Channel FET and P-Channel FET. In case of FET, there is no PN junction between the source and drain. The gate region of FET is made from alternate semiconductor material as compared to the channel between the source and drain terminals.
Difference between BJT and FET
The following table shows the major differences between bipolar junction transistor and field effect transistor.
| Parameter | BJT | FET |
|---|---|---|
| Full form | BJT stands for Bipolar Junction Transistor. | FET stands for Field Effect Transistor. |
| Definition | A type of transistor which uses two types of charge carries viz. electrons and holes for conduction is known as bipolar junction transistor (BJT). | A type of transistor in which electric field is used to control the flow of current in a semiconductor is known as field effect transistor (FET). |
| Drive type | In BJT, the current flow is due to both majority and minority charge carriers. Thus, it is a bipolar device. | In FET, the electric current flows only due to majority charge carriers. Thus, it is a unipolar device. |
| Terminals | BJT has three terminals viz. Emitter, Base and Collector. | FET also has three terminals viz. Source, Drain and Gate. |
| PN junction | BJT consists of two PN junctions viz. emitter-base junction and collector-base junction. | FET does not have PN junctions. |
| Control element | BJT is a current-controlled device. | FET is a voltage controlled device. |
| Types | BJT are of two types: NPN transistor and PNP transistor. | FET are also of two types: N-channel FET and P-channel FET. |
| Configuration | BJT has three configurations: common emitter (CE), common base (CB) and common collector (CC). | FET also has three configurations: common source (CS), common gate (CG) and common drain (CD). |
| Size | BJT is large in size and hence requires more space. Therefore, it is more complicated to fabricate as an IC | FET is comparatively smaller in size. Hence, it is easier to fabricate as an IC. |
| Sensitivity | BJT is more sensitive to the changes in the applied voltage. | FET is less sensitive to the variations in the applied voltage. |
| Relationship between input and output | BJT has linear relationship between input and output. | FET has non-linear relationship between input and output. |
| Thermal noise | BJT has more thermal noise. | The thermal noise in case of FET is much lower. |
| Thermal runway | Thermal runway exits in BJT. | Thermal runway does not exists in FET. |
| Thermal stability | BJT has less thermal stability. | FET has good thermal stability due to absence of minority charge carriers. |
| Input impedance | In case of BJT, the input circuit is forward biased. Thus, the BJT has low input impedance. | FET has high input impedance due to reverse bias of input circuit. |
| Temperature coefficient at high current levels | BJT has positive temperature coefficient. | FET has negative temperature coefficient. |
| Suitability | BJT is suitable for low current applications. | FET is suitable for high current applications. |
| Switching speed | The switching speed of BJT is low. | FET has higher switching speed. |
| Effect of radiation | BJT is susceptible to radiation. | FET is relatively immune to radiation. |
| Gain bandwidth product | BJT has higher gain bandwidth product. | FET has lower gain bandwidth product. |
| Minority carrier storage effect | BJT suffers from minority carrier storage effect. | FET does not suffer from minority carrier storage effect. |
| Cost | BJT is cheaper to manufacture. | FET is relatively expensive to manufacture. |
| Installation | BJT does not require special handling during installation. | FET demands special handling during installation. |
| Applications | BJT is used as switch (in saturation and cut-off region) and amplifier (in active region). | FET is used as switch (in Ohmic and cut-off region) and as amplifier (in saturation region). |
Conclusion
From this discussion, it can be concluded that there are various differences between a bipolar junction transistor (BJT) and a field effect transistor (FET). Both of these transistors are widely used in different electronic applications such as switching and amplification, etc. Thus, the knowledge of these differences can help an individual to select either of them according to the requirements.
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