Sub-nanosecond gated photon counting for high spatial resolution CMOS imagers

Ultra-fast photon counting in standard CMOS imagers can become very challenging at sub-nanosecond temporal resolutions. This is due to the delay and skew of signal propagation along large pixel arrays. A new edge-based solution is presented, circumventing these limitations. This solution is applied to a Single Photon Avalanche Diode based pixel with an embedded analog counter for time gating photon counting. The analog counter has an adjustable sensitivity ranging from 0.1 mV/photon up to 100 mV/photon. This concept was applied to an image sensor with a row length of approximately 15 mm and 330 pixels per row. The pixel counts 55 transistors with a pitch of 45 µm.