High-speed, low-voltage optical receivers consisting of Ge-on-SOI photodiodes paired with CMOS ICs

Silicon-based, monolithically-integrated optical receivers offer the potential of lowering the cost of optical interconnects through simplified packaging and leveraging established Si-manufacturing technology, in addition to enabling new applications such as inter- and intra-chip optical links that will require large-scale receiver arrays. Silicon photodetectors have progressed, and integrated receivers have been demonstrated to operate above 10 Gb/s; however, the poor efficiency of silicon in detecting 850-nm light results in fundamental tradeoffs in performance and/or operating voltage. In contrast, using Ge as the detector material opens the possibility of producing fast, efficient, and low-voltage photodiodes compatible with CMOS processing. We have fabricated planar, interdigitated Ge-on-SOI photodiodes in Ge absorption layers grown directly on SOI wafers. Devices with 10 × 10 μm2 square active areas, biased at -2 V, with an electrode spacing of 0.6 to 0.8 μm, exhibit dark currents less than 10 nA, bandwidths in excess of 23 GHz, and external quantum efficiencies of 52 % (0.35 A/W) at a wavelength of 850 nm. We have built and characterized three different optical receivers using 0.13-μm CMOS ICs: 1) a 15-Gb/s high-gain full receiver (transimpedance amplifier, limiting amplifier, and output driver); 2) a 10-Gb/s, low-power full receiver (powered by a single 1.1-V supply); 3) a 19-Gb/s high-speed receiver front-end (transimpedance amplifier only). These receivers achieve the highest operating speed, highest sensitivity at > 10 Gb/s rates, lowest-voltage single-supply operation, and lowest power consumption for any all-silicon-based receivers reported to-date, and illustrate the performance that can be attained through combining Ge detectors with CMOS analog circuitry.