Silicon photomultiplier

Silicon photomultipliers, often called "SiPM" in the literature, are semiconductor photon sensitive devices built from an avalanche photodiode (APD) array on common silicium substrate. The dimension of each single APD (microcell) can vary from 20 to 100 micrometres depending on the mask used, and their density can be up to 1000 per square millimeter. Every microcell operates in Geiger mode and is decoupled from the others by a polysilicon quenching resistor. Although the microcell works in digital mode the SiPM is an analog device because all the microcells are read in parallel making it possible to generate signals with a dynamic range from a single photon to 1000 photons per square millimeter. The supply voltage (Vb) depends on junction type and varies from a minimum of about 25 V up to 70 V, this being 30 to 60 times lower than the voltage required by a traditional photomultiplier tube (PMT). SiPM have been invented by B. Dolgoshein (Moscow Engineering Physics Institute, Russia) and were initially manufactured by Pulsar (Moscow). Later the design was reproduced by SensL, Photonique, Hamamatsu, and other companies.

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Typically specs for a SiPM:

1. Total quantum efficiency is about 20% being similar to a traditional PMT
2. Gain (G) is also similar to a PMT being approx. 10⁶.
3. G vs Vb curve is linear and not exponential like a PMT
4. Time response is optimized to give an arrival resolution of about 100 ps for single photoelectron
5. Time response is inversly proportional to square root of photoelectrons number within an excitation event
6. Behaviour is independent from magnetic fields
7. Dimensions permits extremely compact, light and robust mechanical design

References

• B. Dolgoshein et al., Nucl. Instr. & Meth., A442(2000)18