The key difference between BJT and FET is that BJT is a current-controlled device, while FET is a voltage-controlled device. This article describes the differences between BJT (Bipolar junction transistor) and FET (Field Effect Transistor).
Before we discuss the differences between BJT and FET, let’s first understand their basics.
What is a Bipolar Junction Transistor (BJT)?
A bipolar junction transistor (BJT) is a three-layer semiconductor device with two p-n junctions. It is used for switching and amplifying electronic signals.
A bipolar junction transistor (BJT) is a semiconductor device composed of three regions: emitter, base, and collector. These regions are connected to three metallic conductors, forming three terminals. The BJT is a current-controlled device, which means a current signal regulates its output.
Based on their construction, there are two types of BJT: NPN transistor and PNP transistor. Both types are widely used in electronic devices for switching and signal amplification.
What is a Field Effect Transistor (FET)?
A Field Effect Transistor (FET) is a type of transistor that contains a semiconductor channel and three terminals: the source, gate and drain. The current flows from the source to the drain, and the voltage signal applied to the gate terminal regulates the current flow between the source and the drain.
Field effect transistors are voltage-controlled devices. The two types of field effect transistors are JFET and MOSFET.
FETs offer several advantages such as low power consumption, high input impedance, faster switching speed, and simple biasing circuit. As a result, they are widely used in modern electronic devices and digital circuits. Now that we have a basic understanding of BJT and FET, let’s compare the two.
Difference between FET and BJT
The table below highlights the main differences between Field Effect Transistor (FET) and Bipolar Junction Transistor (BJT).
| Characteristic | BJT | FET |
| Full form | The full form of BJT Bipolar Junction Transistor. | FET full form is a Field Effect Transistor. |
| Control technology | The flow of current controls a bipolar junction transistor. Therefore, BJT is a current-controlled device. | Field Effect Transistors are electronic devices that are controlled by voltage. Therefore, FET is a voltage-controlled device. |
| Terminals | BJT consists of three terminals: emitter, base, and collector. | FET has three terminals: source, gate, and drain. |
| Charge carries | In a bipolar junction transistor (BJT), the current flows due to both majority and minority charge carriers. | In Field Effect Transistors (FETs), the current flows only due to the majority charge carriers. |
| Device types | BJT is a bipolar device as it allows current to flow through both majority and minority charge carriers. | FET is a unipolar device because the current flows due to the majority charge carriers alone. |
| Input impedance | Bipolar junction transistors (BJTs) have a low input impedance | The FET, or Field Effect Transistor, has a high input impedance. |
| Switching speed | The BJT (Bipolar Junction Transistor) has a slower switching speed compared to other types of transistors. | Field Effect Transistors (FETs) offer faster switching speeds than other transistor types.” |
| Fabrication as an IC | The process of fabricating BJT as an integrated circuit is comparatively complex. | Fabricating FETs as integrated circuits is a simpler process |
| Gain bandwidth product | The BJT (Bipolar Junction Transistor) offers a higher gain bandwidth product. | FET offers a lower gain bandwidth product. |
| Noise | Bipolar junction transistor (BJT) produces more noise | Field Effect Transistor(FET) produces less noise. |
| Types | Bipolar Junction Transistors (BJTs) are available in two types, namely NPN and PNP transistors. | Field-effect transistors (FETs) are classified into two types: N-channel FET and P-channel FET. |
| Voltage drop | The BJT (Bipolar Junction Transistor) has a higher voltage drop than other types of transistors. | FET has a lower voltage drop. |
| Immunity to radiation | The Bipolar Junction Transistor (BJT) is less resistant to radiation compared to other types of transistors. | FETs are more immune to radiation. |
| Thermal stability | The bipolar junction transistor (BJT) has lower thermal stability compared to other transistor types. | FET exhibits superior thermal stability. |
| Size | The BJT transistor is larger in size compared to other types of transistors. | FETs are smaller in size. |
| Energy efficiency | Compared to other semiconductor devices, such as MOSFETs, FETs, Bipolar Junction Transistors (BJTs) are less energy efficient.” | FETs are highly energy efficient. |
| Configurations | There are three configurations of BJT, which are common-base, common-emitter, and common-collector. | FET, or Field Effect Transistor, has three different configurations: common-gate, common-source, and common-drain. |
| Input-output relation | A bipolar junction transistor (BJT) exhibits a linear relationship between its input and output signals. | The input-output relation of FET is non-linear. |
| Applications | BJT is commonly utilized for low-power applications such as switching, amplification, filter circuits, oscillators, etc. | FETs find extensive usage in high-power and high-frequency applications such as digital circuits, integrated circuits, operational amplifiers, measuring devices, oscilloscopes, and RF amplifiers; etc. |
Hence, the major differences between bipolar junction transistors (BJTs) and field effect transistors (FETs) are discussed in this article.