APD Selection Guide - Si or InGaAs

The Active Area of a photodiode impacts its ­ability to ­collect and ­process light. When choosing an APD for range ­finding, one ­always must ­consider its interdependence with the ­component’s ­capacitance and other ­characteristics, such as dark and noise ­current.

In general, Silicon APDs can offer a much larger active area that ­enhances the overall sensitivity and allows for the detection of ­weaker ­return ­signals. This provides greater measurement accuracy, especially at a longer range, and wider field of view (FoV). However, the active area also increases ­capacitance, which in turn ­increases noise in the first-stage ­electrical ­amplification circuitry and slows down the response time of the APD. Larger surface areas also ­introduce larger leakage currents and therefore negatively affect SNR. In contrast, the typically smaller active areas InGaAs APDs result in lower ­capacitance, faster response times and higher bandwidth. 

Therefore, these photodiodes are better suited for applications requiring rapid ­detection and measurement of laser pulses, but their much smaller size does add complexity to alignment of optics to focus the return signal correctly.
 

Dark current in APDs refers to the flow of current in the absence of ­incident light. It contributes to the general background noise, ­reduces the SNR of the APD and has an ­impact on its sensitivity and performance in low-light conditions.

However, dark current levels can vary between different APD ­variants and manufacturing processes, and it is therefore essential to ­consider the specific dark current specifications of each APD model as well as the number of expected received photons for the application and its desired level of performance.

Beyond dark current, the internal avalanche phenomenon itself ­generates noise that is a function of the gain level and quality of the APD. 

Typically, Silicon APDs ­offer a lower noise amplification and thus can reach ­higher ­usable gains, when compared to InGaAs APDs. Given the higher background solar ­illumination and its amplification by ­Silicon APDs, range-finding using InGaAs can often still reach longer ranges.

Capacitance refers to the ability of a device to store electrical charge. In APDs, capacitance arises from the structure and design of the device, ­including the thickness of the depletion region and the associated junction capacitance. The capacitance of an APD affects its ability to respond quickly to changes in incident light intensity, which is crucial for accurate laser range finding.

Silicon APDs, being an indirect material and thus requiring much thicker junctions, therefore typically exhibit lower junction capacitance compared to InGaAs APDs, which is a direct bandgap material and thus much thinner layers suffice to absorb light fully, which allows for faster response times and reduced ­signal distortion. Still, the interdependency ­between the active area and capacitance must be considered and both ­factors balanced.
 

Packaging designs for APDs differ widely and are mostly determined by the requirements of specific applications. Options range from the classical Transistor Outline-cans (TO) and Surface Mounted Devices (SMD) to fiber-pigtailed solutions, devices with lenses to increase the field of view, and ­integrated passband filters tuned to specific ­applications and light sources. It is therefore difficult to state any general ­differences between Silicon and InGaAs packaging technology.