Bipolar Junction Transistor Basics Tutorial

The adage “adapt or perish” holds true throughout the ages, and in the realm of technology, it’s no different. The advent of the point-contact transistor marked the beginning of the electronic technology revolution, completely replacing early vacuum tubes and making electronic devices smaller, more reliable, and more economical. However, not long after, due to its simplicity and stability in manufacturing, the bipolar junction transistor (BJT) replaced it. Despite the development of various transistor structures over the long course of history, the BJT transistor has remained in service.

Point-contact transistors and bipolar junction transistors

BJT Transistor Definition

BJT stands for Bipolar Junction Transistor, also known as a bipolar transistor. It is a three-terminal device with the terminals being the emitter, base, and collector. The term “bipolar” refers to the involvement of two charge carriers (electrons and holes) in the diffusion and drift movement at the PN junction. If only one type of charge carrier is present, it is referred to as a unipolar transistor.

BJT transistors are typically used for switching and amplification. Let’s take amplification as an example to explain its importance.

Electronic products capture external information through sensors and convert it into electrical signals for identification and operation. However, the electrical signals captured by sensors are often very weak and cannot provide enough power to drive the actuator. To address this, we need to construct an amplification circuit to obtain sufficient power to drive the actuator. The BJT transistor is the core of the amplification circuit, controlling energy conversion and faithfully amplifying any small input changes without distortion.

BJT Transistor Symbols

The diagram below shows the symbols for NPN and PNP transistors. How can one identify these symbols?

Symbol of BJT transistor

Observe the symbols: both c and e are indicated by dots, representing the collector and emitter, while b is indicated by a vertical line, representing the base. The arrow in the transistor symbol is designed to point in the direction of current flow. In an NPN transistor, the current flows from the collector to the emitter, while in a PNP transistor, the current flows from the emitter to the collector.

It’s straightforward to recognize, but a reminder is necessary: regardless of whether it’s an NPN or PNP transistor, the arrow’s position is always between b and e. This is an international convention.

BJT Transistor Structure

bjt transistor structure

When manufacturing a BJT transistor, three doped regions, namely the emitter region, base region, and collector region, need to be created on the same silicon wafer. These three regions correspond to the three terminals. During doping, the doping concentration in the three regions is different, with the emitter region having the highest concentration, the collector region having a moderate concentration, and the base region having the lowest concentration.

BJT transistors are classified into two types based on the doping material: NPN-type and PNP-type. Taking a PNP-type transistor as an example, N-type semiconductor is typically doped in the base region to form free electrons, and P-type semiconductor is doped in the emitter and collector regions to form holes. The opposite applies to NPN-type transistors.

PN junctions separate the regions within the transistor, meaning there are two PN junctions inside the BJT transistor. The PN junction between the emitter and base regions is the emitter junction, and the PN junction between the collector and base regions is the collector junction. These two PN junctions form the basis for the operation of the BJT transistor, restricting the free movement of electrons and holes in a static state.

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