## 4 All-Silicon Photodetector Structures
### MBA-Based Devices
In fact, this scaling induces a larger overlap between the optical mode and the implanted region and thus increases optical absorption, reducing the required device length and increasing the frequency response. After a rigorous process of optimization [51, 52], the photodetector proposed by [PERSON] _et al_. had a cross section of about 0.11 \(\upmu\)m\({}^{2}\) showing an internal responsivity of 0.5-0.8 A/W at 1,550 nm, a reverse bias of about 5 V, a leakage current of 2.5 nA/mm and a bandwidth of 10-20 GHz. Further, to increase the device bandwidth, [PERSON] and his co-authors optimized the external circuit components [53] such as the resistance and capacitance of the silicon wings on the sides of the rib wavelength (Figure 8) and on the contact pads. Figure 7: Optical loss **(a)** and photocurrent **(b)**_vs._ post-implantation annealing temperature for an ion-implanted integrated photodiode [48]. Reprinted with permission from [PERSON], _et al_. Copyright © 2006 IEEE. This optimization allowed minimizing the effect of the parasitic components and thus increased the detector bandwidth above 50 GHz. Unfortunately, [PERSON]'s devices have low optical absorption, requiring a diode on the order of milimetres in length to absorb more than 50% of the incoming light. Recently, for the OFC/NFOEC 2010 conference in San Diego, [PERSON] _et al._ proposed a drastic reduction of the device size using a resonance structure [54]; in particular, the light from a straight waveguide coupled into a small-defect Si\({}^{+}\) implanted ring resonator (Figure 9). The ring resonator [55-57] was developed in order to equal the power coupling between the ring and the bus waveguide and the round-trip loss of the ring resonator at the resonance wavelength. In this condition, most of the transmission light is absorbed, increasing electron-hole generation. In Figure 9(a), the transverse section of the ring waveguide is sketched, and the overlap between the region of the optical mode, the defect area obtained and the depletion region of the p-i-n diode is illustrated. In Figure 9(b), a SEM image of the 15-\(\upmu\)m-radius-ring integrated photodetector is reported. The photocurrent of the ring detector under a reverse bias of \(-2\) V is illustrated in Figure 10.  
Figure 8: Top-view optical micrograph **(a)** and schematic cross section **(b)** of the integrated photodiode proposed by [PERSON] _et al._[53]. Copyright © 2009 Optical Society of America. Figure 9: Cross section **(a)** and top view SEM image **(b)** of the fabricated ring detector [54]. Reprinted with permission from [PERSON]. [PERSON], _et al.,_ Copyright © 2010 IEEE. In particular, photocurrents for both before and after a burn in process are reported. The burning-in process, that was also used by [PERSON] _et al_. [52], consists of a forward bias with a current density of 300 mA/cm for 5 minutes and makes possible an increase in the responsivity of the ring photodetector from 0.02 A/W up to about 0.1 A/W at 1,549 nm and \(-\)2 V. Reported leakage current is 0.1 nA at \(-\)2 V of applied reverse bias. The compactness of the device allows reaching a 3-dB bandwidth of about 7 GHz at \(-\)2 V reverse bias. However, all the reported performances of the ring photodetector are relative to the resonance wavelength and thus the C+L bands are not completely covered. To overcome this limitation, [PERSON] _et al_. recently proposed a new ring-resonator-based photodetector [58]. The device was characterized by a responsivity of 0.14 A/W with a reverse bias of \(-\)10 V and a leakage current of 0.2 nA; _i.e._, performance that is comparable with that of [PERSON]'s device. However, the novelty of the proposed structure is the possibility of tuning the photodetector response in a wavelength range from 1,510 nm to 1,600 nm. In particular, resonance wavelength shift has been obtained by thermo-optic tuning of the ring resonance, using a resistive metal strip which overlaps the structure.