Bandwidth and responsivity of photodetector

Bandwidth and responsivity of photodetector
When choosing InGaAs photodetector, everyone wants the same specifications: bandwidth above 10 GHz and responsivity above 0.9 A/W. After flipping through the data manual, I found that these two numbers never appear on the same device. The high bandwidth responsiveness is only 0.5 A/W or even lower, and the high responsiveness bandwidth is only a few hundred MHz. This is not a technical issue with the manufacturer – bandwidth and responsiveness are inherently contradictory in physics, and you can’t have it both ways.
Bandwidth and responsivity are an inherent physical contradiction, rooted in the critical parameter of absorption layer thickness. Increasing the thickness of the absorption layer can improve quantum efficiency (thereby enhancing responsivity), but it will prolong the transit time of charge carriers (thereby reducing bandwidth); Vice versa. Therefore, in the design of standard PIN photodetector, the two cannot be achieved simultaneously and a compromise must be made.
Industry breakthrough plan:
The article introduces three high-end technological solutions aimed at breaking through this contradiction:
Waveguide type detector (WGPD): Decouples the propagation direction of light from the drift direction of charge carriers, and can achieve high bandwidth (>40 GHz) and high responsivity (>0.9 A/W) simultaneously, but the process is complex and the cost is high.
Unidirectional Carrier Transport photodetector (UTC-PD): Utilizing only high-speed electrons for drift, eliminating the transit time limitation of low-speed holes, it can achieve extremely high bandwidth (>100 GHz) and is commonly used in high-speed communication and terahertz fields.
Resonant cavity enhanced photodetector (RCE): Utilizing an optical resonant cavity to enhance light absorption within a thin absorption layer, it can improve quantum efficiency while maintaining high bandwidth, but the operating bandwidth (spectral range) is very narrow.
Suggestions for project selection:
Clarify the priority of requirements: Firstly, determine the minimum bandwidth requirement for the photodetector based on the system signal bandwidth (with a margin of 3 times), and then select the model with the highest responsiveness under this condition.
Pay attention to system level indicators: When evaluating photodetector, attention should be paid to noise equivalent power (NEP) and system sensitivity, not just responsivity, as high responsivity may be accompanied by high noise.
Consider APD photodetector in low-power scenarios: When the incident light power is very low (such as<-30 dBm), the internal gain of avalanche photodiode (APD photodetector) can be used to compensate for the lack of responsiveness, but attention should be paid to its excess noise.
Choosing WGPD with high requirements and high budget: When the system requires both high bandwidth (>20 GHz) and high responsivity (>0.8 A/W), standard PIN detectors cannot meet the requirements, and waveguide type detectors (WGPD) should be directly considered.
Conclusion:
The bandwidth responsivity trade-off of standard PIN photodetector is an inherent physical limitation. To truly break through it, innovation is needed in device structure to physically decouple the light absorption path from the carrier transit path. High end solutions have excellent performance but high costs, so in engineering practice, it is still necessary to make a compromise between specific application scenarios, performance requirements, and budgets.