Over Saturation in SiPMs: The Difference Between Signal Charge and Signal Amplitude
A recent report on the over saturation in SiPMs is puzzling. The measurements, using a variety of SiPMs, show an excess in signal far beyond the physical limit of the number of SiPM microcells without indication of an ultimate saturation. In this work I propose a solution to this problem. Different measurements and theoretical models of avalanche propagation indicate that multiple simultaneous primary avalanches produce an ever narrower and faster signal. This is because of a speed-up of effective avalanche propagation processes. It means that SiPMs, operated at their saturation regime, should become faster the more light they detect. Therefore, signal extraction methods that use the amplitude of the signal should see an over saturation effect. Measurements with a commercial SiPM illuminated with bright picosecond pulses in the saturation regime demonstrate that indeed the rising edge of the SiPM signal gets faster as the light pulses get brighter. A signal extractor based on the amplitude shows a nonlinear behavior in comparison to an integrating charge extractor. This supports the proposed solution for the over saturation effect. Furthermore I show that this effect can already be seen with a bandwidth of 300MHz, which means that it should be taken into account for fast sampling experiments.