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Si photomultipliers with three different guard-ring structures are fabricated, and a detailed comparative study on their device performances is performed. The virtual guard-ring structure shows a high-resolution full width at half maximum in the gamma spectrum and a high breakdown voltage of ~ 66 V but the lowest fill factor of 46.6%-59.8% among the examined structures. The best charge conversion performance, gain, and fill factor (67.1%) are achieved with the trench guard-ring structure. However, this structure shows a low energy resolution, which is supposed to be due to the trench-associated defects. The performance of the N-implantation guard-ring structure is intermediate in most aspects of the device performance compared to the other structures.