Last Updated on October 22, 2023 by Kevin Chen
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Bipolar junction transistors (BJT) and field-effect transistors (FET) are the two most common types of transistors that are used in the field of electronics. They are classified as active semiconductor devices because usually, they are directly connected to the electric current in the circuit. Both transistor types are used for amplification, and switching- the most common functions of transistors in electric circuits.
BJTs and FETs come in different specifications that make them relevant and useful in their respective application areas. These specs include voltage, current capacity, frequency, and package among others. Before you buy a transistor, it will be prudent to confirm such details.
Since bipolar transistors and field effects are transistors and seem to perform the same roles, what makes them different? Does it mean that one transistor is better than the other? And, which one should I choose?
In this guide, we are going to discuss the key differences between BJT and FET transistors. At the end of the article, you will be in a perfect position to choose the right transistor for your application.
Overview of BJT
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They are known as bipolar junction transistors because they use both electrons and holes to operate. The junction is due to the p-n junction that is embedded within the structure of the transistor.
BJTs have two main configurations which are the NPN and PNP. regardless of the configuration or arrangement, a typical BJT is designed to have alternating layers of the p-type and n-type materials. For example, the NPN is configured by sandwiching the p-type material between two n-type materials. The opposite is done with the PNP transistor configuration.
Additionally, BJT transistor has three main terminals, base, collector, and emitter terminals. Each terminal is embedded with a distinct layer meaning that it can have n-type or p-type material. The base terminal of the BJT is lightly doped while the other two terminals (collector and emitter) are heavily doped.
BJT has three regions of operation. These are the Active region, Saturated region, and Cutoff region. At the active region, the transistor will act as an amplifier. Saturated and cutoff regions allow the transistor to work as a switch and in some cases as an oscillator.
A BJT transistor will work when you connect the base-emitter junction in forward bias and the base-collector junction in reverse bias. Electrons will get into the transistor through the base and there will be a smooth flow of current between the collector and the emitter. In this operation mode, the transistor can be used as both switch and amplifier.
Overview of FET
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Field effect transistor is also known as the unipolar transistor. It is named so bearing in mind that only one type of charge carrier can be used at a time. The carrier may be electrons or holes, but not both. There are two main types of FETs: Junction FET (JFET) and Metal Oxide Semiconductor FET (MOSFET). Both types of FETs are used for the amplification and switching functionalities.
Field effect transistors have three terminals: The source, the gate, and the drain terminals. The current flow from the source to the drain. The gate terminal is made of alternate material and is reverse-biased.
There are two main configurations for the FETs; N-channel FET and P-channel FET. The channel in these configurations refer to the path that the electrons use when in transistors. Unlike in the BJT, there are no junctions in the FET.
Like BJT, FET has three regions of operations. These are active, cutoff, and saturated regions. The active region gives the transistor full capacity to work as an amplifier while the two other regions ensure it works as a switch.
Table showing the difference between FET vs BJT
The table below summarizing the difference between BJT vs. FET
| BJT | FET |
| BJT stands for bipolar junction transistor. This means that it has two poles | FET stands for field effect transistor and it has one component hence the name unipolar |
| Comprises three terminals; the base, the emitter and the collector | Has three terminals, the source, the gate and the drain |
| There are two configurations of Bipolar transistors. These are NPN and PNP | There are two configurations of FET. These are JFET, and MOSFET. Both types have the N-channel and P-channel. |
| It is easy to construct BJT | Quite complex to construct as compared to the BJT |
| It is a current-controlled semiconductor component | It is a voltage-controlled semiconductor component |
| The emitter and base terminals of the transistor cannot be interchanged | The source and the drain terminals of the transistors can be interchanged |
| Has high current and voltage gain | Has low current and voltage gain |
| Generates some noise when running in devices | Operates in silent |
| Consumes significantly high power when in operation | Consumes less power hence is energy-efficient |
| BJT is cheaper than FET transistor | FET is more expensive than BJT |
| Larger size than FET | Smaller than BJT and has compact design |
| Perfect choice for applications that have low input current | Ideal for the applications that have low input voltage |
| The transistor has a relatively low switching speed | The transistors have a high switching speed |
| Has high output impedance which leads to a high gain | Low input impedance which translates to a low gain |
| Must have offset voltage | Does not require offset voltage |
| The performance of BJT is highly affected by the frequency changes. | Performance is rarely affected by the frequency changes |
| BJT transistors have a negative temperature coefficient | Have a positive temperature coefficient |
| It highly depends on the input current and this is the reason why it requires high input energy to run | Depends on the input voltage and is the reason why it consumes less energy when running. |
Conclusion
From this detailed guide, you can tell the next direction to take when it comes to buying transistors, whether it is a bipolar junction transistor or a field effect transistor. It is clear that both have been constructed from semiconductor materials and their configuration entails either P-type or N-type. You simply need to analyze the detailed comparison above and make your decision. Even after deciding on a particular type of transistor, ensure that it is of good quality and meets the required safety standards for the electronic components. Buying from reputable transistor manufacturers in China is the surest way to go over this hurdle.
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