The Geiger discharge of a micro pixel is proportional to the applied overvoltage ΔV = VBias – VBreakdown and the microcell capacitance. When hit by a photon, each microcell produces a standardized signal regardless of its position on the SiPM. Due to almost no avalanche fluctuations the excess noise factor is very low and close to 1. Therefore the gain of a SiPM is scaling linearly with the applied overvoltage. Typically the gain of KETEK SiPMs is in the range of 105 to almost 107 (cf. Figure 3). Due to such a high gain typically single photons are already resulting in amplitudes of several mV at 50 Ω load.
- CPixel = Capacitance of Microcell
- ΔV = Overvoltage
- GE = Geometrical Efficiency
- e = Elementary Charge
CD = Capacitance of the micro-Avalanche diode
IPulse = Internal current source representing the Geiger discharge
RQ = Quenching resistor
CQ = Parasitic quenching capacitance
CG = Stray capacitance of all electrical traces
RS = Series resistance
The time constant of the leading signal edge (rise time) is determined by the fired microcell capacitance and the series resistance according to
It is below 1 ns for KETEK SiPMs.
A slow one which is determined by the quenching resistor and the micro cell capacitances according to
A fast one which is determined by the series resistance, the parasitic quenching capacitance and the parasitic grid capacitance. It is only visible as long as the series resistance is small enough.
Figure 16 shows the pulse shape of PM3315-WB and PM3325-WB SiPMs measured with a 5 Ω series / load resistor as well as the exponential fits for the fast and the slow component. The signal rise time is clearly below 1 ns.