The Differences Between Bipolar Junction Transistors And Field-Effect Transistors

BJT vs FET image source Learning about Electronics

When it comes to buying transistors, there are two most common options that you are likely to go for. These are the bipolar junction transistors (BJT) and field-effect transistors (FET). Which one should you go for, and why should you buy it? The only way to answer this question is by drawing a comprehensive comparison between them. Compare the electrical and physical characteristics of the two types of transistors. From there, you will be in a perfect position to know which one is the best option for you. The good news is we are here to help you. In this detailed guide, we are going to analyze the differences between BJT vs. FET transistors. At the end of it all, we will also advise you on which one to buy.

Overview of BJT transistor

A bipolar junction transistor is one of the widely used types of transistors in the market. It is referred to as ‘bipolar’ because it has two materials that support the flow of electric current. Why are the BJT transistors so popular in the electronics market? Their electric properties allow them to be used in a wide range of applications. In other words, it is easy to integrate them into electric circuits where they will be used for performing different roles.

These are the P-type and the N-type semiconductor materials. Electric current can flow from the emitter to the collector and the reverse is true. The transistor also supports two types of charge carriers, which are the holes and the electrons.

The bipolar transistor configuration can either be PNP or NPN. The N and P stand for negative and positive charges respectively. The configuration is determined by the structural arrangement of the semiconductor layers that make up the transistor.

Still, on the physical and structural properties, there are three distinct regions that define a BJT transistor. These are the base region (B), the collector region(C), and the emitter region (E). The base terminal is positioned in the middle of the transistor and serves as the controller. It is a lightly doped terminal of the transistor. Both the collector and the emitter terminals of the transistor are heavily doped and both are next to the base terminal. As the name suggests, the collector serves as a majority charge carrier which can be holes or electrons depending on the configuration of the transistor. On the other hand, the emitter terminal serves as the source of the majority charge carrier.

How do the BJT transistors work?

Like other electronic components, the transistor has to be connected to the current source. From there, the operation is determined by regulating or controlling the flow of current between the base terminals and other terminals. This control action is what is known as transistor biasing. The operation of these transistors is defined by two main modes: Active mode and saturation modes.

In the active mode, the input current flowing through the base terminal is quite small. Using its internal working mechanism, the transistor will ensure that the output current is large enough. There are numerous applications in which BJT transistors operate in the active mode. The most common one is amplification where the output signal is way higher than the input signal.

The second operation mode for transistors is the saturation mode, which is also known as the cut-off mode. As the name suggests, this mode entails the transistor cutting off the flow of electric current or charges after it reaches saturation levels. In other words, the transistor will be serving as a short circuit between the emitter and the collector. In the  the saturation mode, the Vbe is more than 0 and must be greater than the voltage at the collector and at the emitter. This implies that the voltage at the base is higher than the voltage as the collector (Vc) and at the emitter (Ve.

What are the applications of BJT transistors?

BJT transistors have a wide range of application areas which include switching,  amplification, signal conversion, electronic sensors, and motor controllers among others.

Overview of the FET transistors

Field-effect transistors are equally as popular as the BJT transistors only that their structure and operation modes are slightly different. They can also perform some tasks that are designed for BJT such as switching.

Why is it called a ‘field-effect transistor’? The name is derived from the fact that the weak input signal coming into the transistor is used for creating an electric field in the transistor. This electric field has various electrical properties that make the whole transistor useful in its various application areas. For example, the input signal can flip from positive to negative hence controlling the state of the whole circuit.

While BJT transistor is known for its ‘Bipolar’ name, FET is also known as a ‘Unipolar’ transistor. This is mainly because the electric current flowing in the transistor is controlled by one type of charge carrier which can either be holes or electrons and not both.

Types of field effect transistors

Field effect transistors are classified into two broad categories. These are the junction field effect transistors (JFET) and metal oxide field effect transistors (MOSFET). However, there is a third type of FET which is known as an insulated Gate field effect transistor.

Working principle of field effect transistor

The operation of the field-effect transistor is based on the formation of the electric modules between the n-type and pt-type semiconductor materials. This is quite different from the working of the BJT which depends on the flow of electric charges. These electric fields end up forming channels through which electric charges flow. The electric channel can either be p-type or n-type channel. Additionally, the operation and working principle will largely depend on the type of FET, which is either MOSFET or JFET.

The working principle of MOSFET is based on the biasing or application of voltage on the channels that are already existing. The operation can either be in the depletion zone or enhancement zone. In the JFET, the channels are created during the biasing process.

What is the difference between BJT and FET in tabular form?

Here is a table that gives a summary of the differences between BJT and FET

Conclusion

This article covers all the important details that you should know regarding the differences between bipolar junction transistors and field effect transistors. Based on this knowledge you can make the right decision on which transistor to use in your application.

Once you decide on the type, the next step is choosing where to buy the best transistor for your application. This will have an impact on the quality and reliability of the transistor that you will end up with. It is always advisable to buy through reputable electronic components sourcing agents in China. They have the links to the best transistor manufacturers and suppliers.

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