Depth-dependent inter-pixel distance in trench-isolated LGADs for shallow radiation detection

The fill factor of finely segmented low-gain avalanche detectors (LGAD) is limited by the effective inter-pixel distance (IPD), particularly when charge is generated far from the gain layer. In this work, 200 μ m thick trench-isolated LGADs (TI-LGADs) fabricated by Micron Semiconductor Ltd. were characterized using single-photon and two-photon absorption transient current technique measurements at ELI Beamlines to determine the dependence of IPD on interaction depth, illumination direction, and gain-layer doping. The measurements show that the effective IPD is smallest near the trench side of the device and increases as charge generation occurs deeper in the sensor bulk, with a minimum measured IPD of approximately 7 μ m. Devices with higher gain-layer doping exhibit larger IPD, indicating a trade-off between gain and effective fill factor, while depth-resolved measurements reveal enhanced charge collection at the trench edge consistent with a localized high-field region at the gain-layer/trench interface. These results show that, in thick TI-LGADs, back-side shallow absorption corresponds to charge generation far from the multiplication region and can therefore lead to an increased IPD, which may limit the performance of small-pixel designs. The findings identify an important design constraint for TI-LGADs intended for applications involving shallow energy deposition under back-side illumination.