# The Frequency Response Characteristics of Ge-on-Si Photodetectors Under High Incident Power
## 4 Conclusions  
This work investigated the mechanism underlying the bandwidth reduction in germanium photodetectors at high incident light power. According to experimental and simulation results, we conclude that the primary reasons for bandwidth reduction at high incident light power are the carrier-shielding effect and the decrease in the carrier saturation drift velocity. The carrier migration rate increases with the increase in electric field strength until it reaches the saturation mobility. The carrier-shielding effect reduces the electric field inside the absorption region, leading to a decrease in the carrier migration rate. The reduction in the saturation carrier mobility, which is the maximum migration rate of carriers, determines the highest rate at which carriers can migrate. We conducted multiple tests on the data in the paper and the trend of the test results shows good consistency. Each data point used in our paper is the median of multiple tests. For incident light powers below 2.5 mW, despite a notable carrier-shielding effect, the electric field intensity across the absorption region remains sufficient for carriers to reach their saturation drift velocity. Thus, the carrier-shielding effect has a minimal impact on the bandwidth of the photodetector in this range. Instead, the primary factor influencing bandwidth is the increased concentration of photogenerated carriers, which leads to a reduction in the saturation drift velocity of the holes. When the incident light power exceeds 3 mW, the electric field intensity in the carrier-shielding region becomes too weak to maintain the saturation drift velocity of the carriers. By increasing the applied bias voltage, we mitigate the effect of carrier shielding on the electric field intensity. However, the bandwidth remains lower than that observed at 0.5 mW. Therefore, an increase in the concentration of photogenerated carriers leads to a decrease in the saturation drift velocity of the holes, which is another significant factor in bandwidth reduction. Figure 6: The tested and calculated normalized S\({}_{21}\) response of the photodetectors under different incident light powers. The main reasons for the decrease in bandwidth under high incident light power are the carrier-shielding effect and the decrease in carrier saturation drift velocity, both of which are caused by excessively high distributions of local carrier concentration. Therefore, in the design of germanium photodetectors, owing to the coupling of evanescent waves, the light field in the germanium absorption region exhibits a strong-weak alternating distribution. This leads to a carrier-shielding effect and a decrease in the carrier saturation drift velocity in regions with strong light field distributions, thereby affecting the bandwidth. Optimizing the structural design to make the light field distribution more uniform can effectively improve the frequency response of germanium photodetectors under high incident light power. The photodetector presented in this study did not demonstrate excellent saturation performance. The saturation characteristics of the device could be further improved by optimizing its design to enhance the uniformity of the light field distribution within the germanium absorption region. Conceptualization, [PERSON] and J.C.; methodology, [PERSON]; software, [PERSON]; validation, [PERSON] and J.C.; formal analysis, [PERSON]; investigation, [PERSON]; resources, [PERSON]; data curation, [PERSON]; writing--original draft preparation, [PERSON]; writing--review and editing, [PERSON]; funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript. This research was funded by National Key Research and Development Program of China, grant number 2022 YFB2803700, Natural Science Foundation of Fujian Province, grant number 2023J011030 and 2023J011029, the Fundamental Research Project of Shenzhen Science and Technology Fund, grant number JCYJ20220818103408017, JCYJ20220818103408018, JCYJ20230807154759002 and JCYJ20230807153859005.