Dead Time Compensation in CMOS Single Photon Avalanche Diodes With Active Quenching and External Reset

Single photon avalanche diodes (SPADs) in CMOS are becoming increasingly interesting devices for timing applications, such as fluorescence lifetime imaging, positron emission tomography, and time of flight mass spectroscopy. The CMOS allows integration of functionalities like time-to-digital converters within the same pixel, and the manufacturing of large format arrays. Dead time has to be taken into account in order to correctly interpret SPAD measurements. In this paper, we derive and test a model for dead time in real SPADs where reset is generated off-pixel. We test the model using our own custom designed devices made in a low-voltage 180-nm CMOS image sensor process with full custom implants. A Monte Carlo simulation is implemented to compare with experimental results. Using a fitting method, higher values of the photon detection efficiency (PDE) can be extracted than with a simple linear fit. The resulting PDE corrections are significant, up to 100% depending on the conditions. The limitations are approximated, and it is found that accurate predictions of the true count rate are possible over a control range of 0.25-1.0 MHz.