In this blog post, we will learn about the principle and importance of avalanche photodiodes, which effectively amplify weak light signals in optical communications.
Optical communications use light, which allows information to be transmitted very quickly. However, as the length of the optical communication cable increases, the intensity of the light decreases, so the optical signal received in long-distance communications can become very weak. This is a physical characteristic: since light is transmitted through photons, weak light intensity means that fewer photons reach the receiver per unit time. Therefore, devices that detect the reduced number of photons are essential in optical communication, and avalanche photodiodes are widely used as semiconductor devices that convert weak optical signals into electrical signals that can be measured.
The quality of optical fibres and their installation methods are also important for improving the efficiency and reliability of optical communication systems. High-quality optical fibres minimise signal loss and reduce damage caused by environmental factors. In addition, even minor damage or bending during the installation of optical fibres can cause signal loss, so precise installation techniques are required.
For example, submarine optical cables are thousands of kilometres long and are designed and installed to withstand deep-sea pressure and ocean currents. These optical cables play a pivotal role in international data communication and account for a significant portion of global Internet traffic. Avalanche photodiodes consist mainly of an absorption layer, an avalanche region, and electrodes.
When photons with sufficient energy enter the absorption layer, electron (-) and hole (+) pairs can be generated. The number of electron-hole pairs generated relative to the number of photons entering is called the quantum efficiency. Quantum efficiency, which is determined by the characteristics of the device and the wavelength of the incident light, is one of the important factors affecting the performance of avalanche photodiodes.
The electrons and holes generated in the absorption layer move to the positive and negative electrodes, respectively, and in this process, the electrons pass through the avalanche region. In this region, a strong electric field exists due to the reverse voltage applied to the electrodes of the device, and this electric field increases as the reverse voltage increases. In this region, the electrons are rapidly accelerated by the strong electric field and reach high speeds. After reaching sufficient speed, the electrons collide with the atoms that make up the semiconductor material in the avalanche region, slowing down and creating new electron-hole pairs. This phenomenon is called collision ionisation. The newly generated electrons and existing electrons are accelerated again in the avalanche region until they reach the electrode, repeating collision ionisation. The resulting large increase in the number of electrons is called ‘avalanche multiplication,’ and the degree of increase in the number of electrons, i.e., the number of electrons emitted from the electrode per electron entering the avalanche region, is called the multiplication factor. The multiplication factor increases as the electric field strength in the avalanche region increases and as the operating temperature decreases. The magnitude of the current is proportional to the number of electrons flowing per unit time. Through this series of processes, the intensity of the light signal is converted into the magnitude of the current.
On the other hand, the wavelength band of light that can be detected by avalanche photodiodes differs depending on the semiconductor material that constitutes the absorption layer and the avalanche region. For example, silicon can detect light in the 300 to 1,100 nm wavelength band, which mainly corresponds to the visible and near-infrared regions. Germanium can detect light in the 800 to 1,600 nm wavelength range, which mainly corresponds to the near-infrared and mid-infrared regions. By using various semiconductor materials, avalanche photodiodes can be designed for a wide range of applications. For example, avalanche photodiodes for communications are mainly made of silicon, while semiconductor materials such as germanium are used in the military and space exploration fields. These photodiodes are designed to deliver optimal performance according to their respective characteristics.
Recently, research is actively underway to develop more efficient and sensitive avalanche photodiodes. Examples include the development of new materials using nanostructures and the introduction of new alloys that overcome the limitations of existing semiconductor materials. Such technological advances will dramatically improve the performance of optical communications and further enhance the quality of long-distance communications. These technologies are becoming increasingly important as they can be used in various fields such as medical, military, and space exploration, in addition to optical communications.
Currently, various types of avalanche photodiodes are being manufactured and used to meet the diverse needs and requirements of users. In particular, high-efficiency avalanche photodiodes are essential in fields that require high-speed data transmission due to the development of optical communications. With future technological advances, the performance of avalanche photodiodes is expected to improve even further. For example, next-generation avalanche photodiodes combined with nanotechnology are expected to have higher quantum efficiency and multiplication coefficients than existing ones. This will bring about innovative changes in various fields, such as long-distance communication, optical sensors, medical imaging, and precision measurement.
With the advancement of optical communication technology, the role of avalanche photodiodes is becoming increasingly important, and these devices will play a key role in the information society of the future. Combined with advanced technology, avalanche photodiodes will bring about major changes in our daily lives and, furthermore, revolutionise the transmission of information worldwide.
