Waveguide Photodiodes: New Technology Supporting Data Center Speed

Data Centers and Optical Transceivers Evolution

Data center traffic around the world is rapidly increasing. This growth is driven by AI/ML development and implementation, the spread of cloud services, and the increasing user base for video streaming and AR/VR content. These trends are clearly expected to continue. To address this situation, backbone optical communication networks require further enhancement in capacity and transmission speeds.

To provide stable optical communication services to users, infrastructure networks and data centers need networks that operate at higher speeds with minimal delay. Optical transceivers are critical components in this process. Traditionally, optical transceivers operated at 100Gbps as the standard, but in response to network acceleration requirements, high-speed products at 400G and 800G began entering the market around 2014. Looking toward the future, the speeds required for optical transceivers are expected to reach 1.6T and 3.2Tbps.

Photodiode Basic Structure and Challenges

Optical transceivers are devices that mutually convert between electrical signals and optical signals. In the transmitter section, semiconductor laser diodes convert electricity to light. In the receiver section, photodiodes (PD) convert optical signals into electrical signals. While the basic structure and operating principles of photodiodes are explained in detail in our previous article, this article specifically addresses the technical challenges related to high-speed operation.
Waveguide photodiodes are also devices that convert optical signals transmitted through optical fibers into electrical signals. As the name suggests, they are components that combine a “waveguide” path that guides light with a “photodiode” that converts light to electricity. While the basic structure and operating principles of photodiodes are explained in detail in our previous article. This article focuses specifically on challenges related to high-speed operation. The major difference between conventional photodiodes and waveguide photodiodes lies in their physical structure.
The figure below shows the structure of a typical photodiode (pn type).

Conventional pn-type photodiodes have a structure with a p-type semiconductor layer containing many holes (particles with positive charge) on top and an n-type semiconductor layer containing many electrons with negative charge at the bottom. A layer called the “depletion layer” forms at their junction, and light is primarily absorbed near this area.

Pn-type photodiodes have a relatively simple structure that makes manufacturing comparatively easy. However, this simple structure presents challenges for achieving high speed performance.
The key factor in the high-speed operation of photodiodes is “bandwidth.” Bandwidth represents the number of signals that can be processed per second, and a larger bandwidth allows more information to be processed in a shorter time. For example, with a bandwidth of 1GHz, 1 billion signal operations are possible per second.

In pn-type photodiodes, the light-absorbing region (depletion layer) needs to be thick for efficient light absorption. This is because the longer the distance light travels through the semiconductor, the more light can be absorbed. However, when the depletion layer is thickened, the travel distance for the electrons and holes generated by light increases, resulting in slower device response time.

The Innovative Structure of Waveguide Photodiodes

To address this problem, waveguide photodiodes with a novel structure were conceived. With a design that enables efficient light absorption and carrier movement, waveguide photodiodes can achieve both high-speed operation and high sensitivity.

In conventional photodiodes, light enters from the top of the component, and semiconductor layers that absorb light, such as the depletion layer, convert that light into electricity. In contrast, with waveguide photodiodes, light enters from the side and is gradually absorbed while passing through a horizontally elongated layer called a “waveguide” made of compound semiconductors, converting to electricity.

The concept can be compared to rain falling directly onto the ground from above (conventional photodiodes) versus water (or in reality, light) flowing through a long horizontal pipe (waveguide type). This horizontal light propagation feature allows the light to travel a distance of tens to hundreds of micrometers within the absorption layer, significantly improving light absorption efficiency. The ability to create thin waveguide layers is also a critical advantage. With thinner waveguide layers, the travel time for electrons and holes generated by light to reach the electrodes is reduced, enabling faster response. Additionally, the ability to make thinner devices facilitates integration with other optical components on the same substrate, contributing to component miniaturization.

Thus, waveguide photodiodes are a technology that can achieve both high light absorption efficiency and fast response speed. However, there are still several challenges for practical implementation, such as efficiently coupling light into fine waveguides. The follow-up article introduces our initiatives to address these challenges and provides more detailed application examples.

At Dexerials, we are committed to the realization of next-generation high-speed optical communications through the development of waveguide photodiodes.