What is a Transistor and How Does It Work

In the expansive realm of technology, even the intricacies so minute that they escape the naked eye play a crucial role. Among these diminutive components, transistors undoubtedly emerge as stars. Although individually they may not be eye-catching, they often exist in the billions, working in pairs within electronic devices. In this article, TechSparks delves into the fundamental knowledge of transistors, aiming to provide you with a better understanding of this cornerstone in the world of technology.


What is a Transistor

Broadly speaking, the term “transistor” encompasses all individual components based on semiconductor materials, including diodes and triodes. Traditionally, when we mention transistors, we are typically referring to crystal triodes.

These transistors can exist individually on printed circuit boards or be integrated into IC chips to function as a whole. With the rapid advancement of semiconductor technology, the size of transistors has gradually reduced. On chips with an area of less than a hundred square millimeters, we can see the integration of tens of millions or even billions of transistors, and this number continues to grow exponentially. According to Moore’s Law predictions, the number of transistors in a chip will double every 18 to 24 months. This ongoing innovation enhances the performance and functionality of chips, significantly propelling the development of the microelectronics era.

Further Reading: Is Smaller Always Better for Transistor Size?

Transistors on PCB vs transistors on chip

In the field of computer science, transistors provide the foundation for faster and more reliable computing, laying a solid groundwork for information processing. In information technology, transistors enable the processing, transmission, and storage of information in binary form, playing a crucial role in the formation of an information society. In the communication sector, their application makes communication devices more advanced and efficient. In power systems, they strengthen the control and conversion of electric power, contributing to energy conservation.

Some advantages of transistors

Evolution of Transistors

The birth of transistors can be traced back to the early 20th century when Bardeen and Brattain first developed the point-contact transistor, marking the beginning of the transistor era. The first-generation transistors used germanium as a semiconductor material, forming a structure with two metal points in contact with the semiconductor material. Although this design may seem rudimentary now, it was a significant technological breakthrough at the time, enhancing the functionality of electronic products and reducing manufacturing costs, greatly driving the development of the electronics industry.

The most widely used in modern electronics is the silicon, further divided into BJT transistors and FET transistors based on internal structure and operating principles:

  • BJT Transistors: Control current based on PN junctions.
  • FET Transistors: Control current based on electric field.

In addition to these, there are more types of transistors available for designers to choose from, depending on the classification criteria. From any perspective, modern transistors not only have more powerful and reliable functionality but also contribute to large-scale production.

Looking ahead, transistor technology will continue to evolve, continuously improving performance, reducing size, and becoming more cost-effective. Potential future trends include nanotechnology, quantum technology, three-dimensional integration, and innovative structural designs and upgraded material utilization.

Transistors Under the Microscope

For those unfamiliar with electronics, it might seem like an exaggeration to claim that a chip the size of a thumbnail contains billions of transistors. Let’s magnify the chip continuously to witness this awe-inspiring scene.

The figure below shows a cross-sectional view and a top view of a CPU captured under a scanning electron microscope. You will notice that this CPU has a 10-layer structure, and as we go deeper into the layers, the line width becomes narrower. This is because the bottom layer consists of devices scattered with transistors. These transistors are not installed inside the chip but carved on it using technologies such as lithography, etching, and deposition.

cross-sectional view and a top view of a CPU

Working Principle of Transistors

The working principle varies based on the structure of the transistor. The typical BJT transistor controls current by introducing dopants such as 3-valent boron or 5-valent phosphorus into semiconductor materials to form N-type and P-type regions, controlling the current similarly to the diode’s working principle. Unlike the diode, the transistor is divided into three parts: the base (B), the collector (C), and the emitter (E). Depending on different combinations, the transistor can be divided into NPN-type and PNP-type.

In the circuit below, the transistor is off. This means no current can pass through it, and the light-emitting diode is also off.

Example circuit with transistor turned off

To turn on the transistor, we need to apply a forward voltage between the base and emitter, typically around 0.6V to 0.7V. To achieve this, we can connect a battery to ensure that its voltage is sufficient to overcome the potential barrier.

Additionally, we can consider the transistor as two back-to-back diodes. This means that by adding a resistor in series, we can generate a voltage of 0.6V to 0.7V across the resistor, thereby turning on the transistor.

Finally, to control the transistor more flexibly, we can add a push-button switch. By pressing the button, we can change the circuit connection status, thereby controlling the conduction of the transistor and influencing the LED’s on/off state.

Control transistors via key switches

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