Gain suppression mechanism observed in Low Gain Avalanche Detectors

Low Gain Avalanche Diodes (LGADs) is one of the candidate sensing technologies for future 4D-tracking applications and recently have been qualified to be used in the ATLAS and CMS timing detectors for the CERN High Luminosity Large Hadron Collider upgrade. LGADs can achieve an excellent timing performance by the presence of an internal gain that improves the signal-to-noise ratio leading to a better time resolution. These detectors are designed to exhibit a moderate gain with an increase of the reverse bias voltage. The value of the gain strongly depends on the temperature. Thus, these two values must be kept under control in the experiments to maintain the gain within the required values. A reduction in the reverse bias or an increase in the temperature will reduce the gain significantly. In this paper, a mechanism for gain reduction in LGADs is going to be presented. It was observed, that the gain measured in these devices depends on the charge density projected into the gain layer, generated by a laser or a charged particle in their bulk. Measurements performed with different detectors showed that ionizing processes that induce more charge density in the detector bulk lead to a decrease in the detector’s measured gain. Measurements conducted with an IR-laser and a Sr-90 in the lab, modifying the charge density generated in the detector bulk, confirmed this mechanism and will be presented here.