# Review of CMOS Integrated Circuit Technologies for High-Speed Photo-Detection
## 1 Introduction  
Today, people's everyday lives are connected online, i.e., anyone can access or share data worldwide at any time and place. For example, most people today are familiar with accessing streaming media from YouTube, and using social network services and cloud services for personal purposes. The concept of internet of things (IoT) is not new anymore, because IoT devices have already become a part of people's lives. Two important factors contributing to this revolution are the development of high-performance computing systems and data communication systems which are closely interdependent. Especially, the requirement of high-performance data communication systems is being emphasized more than ever, because of ever-increasing demand for data throughput in every computing system. Even a microprocessor and memory constituting a tiny computing system is now required to handle several hundreds of gigabits per second, and it is growing at a relentless rate. A similar phenomenon is observed in longer-distance applications such as inter-server and long-haul communications. The forecast of data throughput demands is provided by Cisco, one ofthe leading companies specializing in networking equipment. Figure 1 forecasts the annual global IP traffic by 2020 with a compound annual growth rate of 20%, predicting that the IP traffic would become 200 exabytes/month in 2020 [1]. It is worth investigating the main factors that result in these explosive total data throughput demands. Figure 2a illustrates the breakdown of the total global IP traffic by application. As shown in this figure, the proportion that internet video applications account for will increase in the future. This is reasonably explained in Figure 2b by further subdividing video applications into Standard Definition (SD), High Definition (HD), and Ultra-High Definition (UHD). From these investigations, it is evident that high-quality video contents would be dominant in future IP traffic because of increasing demand for 3D media content and virtual reality. It can also be predicted that the capability of handling high data bandwidth will be required for not only data centers but also end-user devices. The above data predicts that the bandwidth requirement will inevitably increase for both data centers and end-user devices. Another important requirement is the reduction of power consumption. Figure 3a shows the power breakdown of typical data centers, indicating that the current communication systems are consuming much power in handling extremely high data rates so that a significant part of resources is being wasted for cooling systems. In the case of end-user devices, the proportion of mobile devices will exceed that of PCs as shown in Figure 3b, meaning the importance of power management will be undoubtedly more pronounced in the future. In summary, it is required to build a reliable communication system satisfying the aforementioned requirements, necessitating innovations in the existing communication systems. The primary obstacle that hinders high-speed operation is the electrical limitation of the copper-based interconnection which is predominantly used in most applications except for long-reach  
Figure 1: Forecast of total global IP traffic [1]. Figure 2: Forecast of global IP traffic (**a**) by application and (**b**) by video content [1]. interconnections. The frequency response of a copper wire severely degrades due to conductor loss and dielectric loss at higher frequencies. Channel reflection and crosstalk are also detrimental to high-speed operation, limiting the maximum operating frequency to only several GHz over a few tens of meters. Even if various circuit techniques have successfully overcome this physical limit, copper-based interconnection faces the challenge of bandwidth limit as the data rate exceeds several tens of Gb/s. On the other hand, optical interconnection can provide a much higher bandwidth and is free from reflection and crosstalk.