Learn the basics of silicon photomultipliers

Marek Gali
Marek Gali, January 2018

Silicon photomultipliers (SiPMs) are innovative optical sensors for the detection of ultra-low light levels down to single photons. Learn a basic understanding of the technology and its capabilities.

The silicon photomultiplier (SiPM) is a radiation detector with extremely high sensitivity, high efficiency, and very low time jitter. It is based on reversed biased p/n diodes and can directly detect light from near ultra violet to near infrared. SiPMs are employed in all those applications where low light/radiation levels must be measured and quantified with high precision.

Matrix of microcells

A SiPM consists of a matrix of small-sized sensitive elements called micro-cells (or pixels) all connected in parallel. Each micro-cell is a Geiger-Mode avalanche photo-diode (GM-APD) working beyond the breakdown voltage and integrating a resistor for passive quenching.

blog_SiPM-micro-cell.jpg

Operation modes

In quiescent mode, the diode is reversed biased and no current is flowing. When a photon is absorbed avalanche multiplication is ongoing inside the GM-APD. Once triggered, the avalanche process is self-sustaining meaning that, without quenching, a steady current flows indefinitely in the device. Now, the avalanche process is quenched and the GM-APD enters the recovery phase and returns in the quiescent mode, ready for the detection of a new photon.

Output pulse

During recovery, the GM-APD cannot detect other photons. SiPMs overcome this limitation thanks to the parallel arrangement of several micro-cells. When N photons are detected (which means that N photons arrive on N different micro-cells producing N single-cell signals) the SiPM output pulse is N-times larger than the single-cell response. In that, the N independent current pulses just add up at the SiPM terminals. Note that both the amplitude and the area of each SiPM pulse, which is the total charge delivered by the detector, are proportional to the number of detected photons. If the number of incoming photons is larger than the number of micro-cells in the SiPM, saturation occurs and neither the amplitude nor the area of the output pulse can give information on the number of incoming photons anymore.

Read the complete White Paper "Introduction to silicon photomultipliers" and …

1. … learn more about the use of SiPMs with scintillating crystals i.e. in spectroscopy applications.
2. … understand the breakdown voltage and how it influences the SiPM performance.
3. … find out more about SiPM photon detection efficiency, gain and different sources of noise.

 White paper: SiPMs

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