# Review of CMOS Integrated Circuit Technologies for High-Speed Photo-Detection
## 2 Silicon Photonics for High-Speed Data Communications
### Silicon Photonics
The driver further increases the amplitude of the data signal to make an optimum input condition for the MZI. The MZI is based on a reverse-biased p-n junction for high-speed operation at the cost of reduced phase shift, thus reducing the extinction ratio (ER). ER can be raised by increasing the length of the modulator or enlarging the driver output swing. The driver output swing and the length of the modulator in this design are 5 V\({}_{\text{ppd}}\) and 4 mm, respectively. A holographic lens is used as a fiber-optic coupler for both the TX and the RX. The PD at the RX is flip-chip bonded to the silicon die. The transimpedance amplifier (TIA) converts the PD current to a voltage which is further amplified by a limiting amplifier (LA) for the proper operation of the CDR. Following the CDR, an output buffer is employed to drive a differential 100-\(\Omega\) transmission line. In 2007, Luxtera further developed their photonic device library by adding a dense WDM (DWDM) building block. By implementing DWDM multiplexers and demultiplexers, they succeeded in integrating a 4 \(\times\) 10-Gb/s DWDM optoelectronic transceiver in a 0.13-\(\upmu\)m CMOS SOI technology [45]. The silicon photonics research group at MIT has also published successful works in monolithic optical transceivers. In 2012, the first monolithic optical receiver in a sub-100-nm standard SOI process was implemented [46]. In this work, novel approaches are proposed toward implementing a PD and a detecting scheme, which can be attributed to the increased design flexibility of the PD in the monolithic integration platform. An important advantage of the monolithic integration is that low PD and wiring capacitances can be obtained. An integrating receiver illustrated in Figure 7 can be the most appropriate topology by exploiting the low-capacitance condition and the presence of an RX clock. However, an insufficient timing margin for evaluation can severely degrade the sensitivity. PD splitting combined with a double-data-rate (DDR) scheme is implemented in this work to alleviate the timing margin. By interdigitating the metal contacts of the PD, it can be divided into two separate PDs sharing a common waveguide, thus reducing the photocurrent of each path by half. In spite of the halved photocurrent, the PD splitting is more advantageous at higher data rates considering that the sensitivity degrades exponentially with the shortened evaluation time. Another improvement is at the PD structure that incorporates a ring resonator to confine the light, thereby enhancing the absorption with a smaller PD length. The implemented receiver chip operates at the data rate of up to 3.5 Gb/s  
Figure 6: 2 \(\times\) 10-Gb/s dual channel optoelectronic transceiver [44]. with an extremely high energy efficiency of 50 fJ/b. This demonstration is significant in that it paved the way for future memory-processor interfaces based on silicon photonics. A monolithic optical transceiver operating at higher speed was also reported jointly by UC San Diego and Oracle in the same year [47]. The chip is implemented in a 0.13-\(\upmu\)m SOI CMOS technology and achieves an operating speed of 25 Gb/s with an energy efficiency of 10.2 pJ/b for the entire transceiver. The optical TX is based on a micro-ring modulator and the optical RX employs a Ge photodetector. Notably, an asymmetric pre-emphasis is used even for the reverse-biased ring modulator. When the optical devices are forward-biased, or more specifically when operating a VCSEL or a ring modulator in a forward-biased condition, the storage time of minority carriers obstructs the operating speed and causes asymmetric rising and falling times. On the other hand, in the case of a reverse-biased ring modulator, the photon lifetime dominates the maximum operating speed, which also incurs asymmetry. Therefore, the asymmetric pre-emphasis, or independently controlling the rising and falling edges, is essential even for the reverse-biased condition in order to improve the signal quality at higher speed.