Free-Running GaN Ultraviolet Single-Photon Avalanche Diodes With Robust Avalanche Breakdown

We report free-running GaN ultraviolet (UV) single-photon avalanche diodes (SPADs) for the first time. The device exhibits an ultra-low dark current density ( ${J}_{\text {BR}}\text {)}$ below $3\times 10^{-{5}}$ A/cm2, a record-high avalanche gain exceeding $5\times 10^{{7}}$ , and stable avalanche behavior over a wide temperature range from 100 K to 600 K, with a positive temperature coefficient of 0.017 V/K. Notably, the avalanche current rises abruptly beyond the breakdown voltage ( ${V}_{\text {BR}}\text {)}$ , increasing by more than six orders of magnitude within a 2 V range. This robust avalanche behavior enables spontaneous avalanche initiation without the need for external pulsed-voltage triggering, thereby achieving free-running operation. Furthermore, the avalanche current can be passively quenched by a low-resistance resistor, which ensures short RC time constants and minimal parasitic effects. Consequently, the device delivers a fast counting process of several nanoseconds, a high photon count rate (PCR) exceeding 6 MHz, and a single-photon detection efficiency (SPDE) above 3%.