KETEK's SiPM data sheets detail key performance parameters of their silicon photomultipliers and Noise effects are a major consideration when choosing the optimum detector for your application. Noise in a silicon photomultipliers can be considered as four distinct characteristics:-
- Dark Count Rate
- Optical Crosstalk
- Correlated Noise Probability
Dark Count Rate:
Spontaneous breakdown of a Geiger-mode microcell triggered by thermally generated electrons will release the same charge as when a photon is detected. These breakdowns are termed "Dark Counts" and are indistinguishable from actual photon counts. The frequency of these events is termed the "Dark Count Rate" (DCR) and the sum of all of the dark counts generates the "Dark Current" of an SiPM. Two factors influence the DCR for a given SiPM, the overvoltage and the temperature.
Increasing Overvoltage will increase the breakdown probability and therefore the DCR.
Increasing temperature will increase the Carrier Density in silicon, as a rough guide DCR doubles for every 9°C increase in temperature.
During the Geiger discharge of a microcell an average of 3 secondary photons (450nm-1600nm) are created per 105 avalanche electrons. This equates to between 3 and 50 secondary photons per discharge which have the potential to cause Geiger discharge of surrounding microcells via three mechanisms:-
(1) Directly to a neighbouring cell, known as "direct optical crosstalk" this is by far the dominant contributor to DCR in KETEK SiPMs.
(2) A secondary photon is able to generate an electron-hole-pair close to a neighbouring cell. The carriers can diffuse to the microcell and cause their discharge, known as "delayed optical crosstalk". Delayed Optical Crosstalk is considered almost negligible in KETEK SiPMs and is less than 0.1% (see below).
(3a, b) Secondary photons can be reflected at one of the various interfaces (i.e. package boundary or SiPM backside) and reach a neighbouring cell via an indirect path, known as "indirect optical cross talk".
A key feature of KETEK SiPMs featuring 25µm or larger microcells is a deep trench between the individual microcells which provides optical isolation. Direct Optical Crosstalk is significantly suppressed by the trench - whilst the deeper the trench the more significantly it can reduce delayed optical crosstalk.
Optical Crosstalk probability increases with overvoltage as demonstrated in this graph - which includes the <0.1% contribution from Indirect Optical Crosstalk.
During the microcell's breakdown and avalanche process charge carriers can become trapped and then released with a delay of up to several µs. KETEK SiPMs show very low afterpulsing of less than 1%.
Correlated Noise Probability:
After a microcell has been fired, there is a certain probability for another breakdown after the inital one. Correlated noise includes afterpulsing and delayed crosstalk and is very low for KETEK SiPMs as demonstrated by this graph of Correlated noise probability including afterpulsing and delayed crosstalk for WB-Series SiPMs.