An 8.5Gb/s CMOS OEIC with on-chip photodiode for short-distance optical communications

Recently, low-cost silicon optoelectronic integrated circuits (OEICs) have been drawing attention for applications in short-distance optical communications such as chip-to-chip and board-to-board interconnects, LAN, data storage networks, etc [1–4]. Particularly, single-chip OEICs with on-chip silicon photodiodes provide a number of advantages including low cost, reduced ground-bounce, and bond-wire-induced coupling. Nevertheless, the slow response of silicon photodiodes in a standard CMOS process serves as a major bottleneck for high-speed communication [1]. To improve the bandwidth of silicon photodiodes, either some process modification or avalanche photodiode implementation has been developed. However, the former results in increased costs, whereas the latter has reliability issues. Although a differential photodiode configuration was originally proposed for bandwidth extension [2–4], the operation speed is still limited to several-hundred Mb/s. Meanwhile, the bandwidth can be extended by exploiting equalization filter [1, 3]. For relatively low-Gb/s operations, fixed equalization filter is sufficient, because photodiode responsivity is dominantly determined by diffusion currents which are not sensitive to process and temperature variations. For higher speeds, the responsivity becomes strongly dependent on the process and temperature variations, because it is mainly determined by the carrier mobility. Thereby, equalizers for high-Gb/s optical receivers require an adaptation algorithm to compensate the significant process and temperature variations. In this paper, an OEIC with on-chip photodiode is presented. Bandwidth and responsivity are compensated by a compact adaptive equalizer, thus achieving 8.5Gb/s operation.