Panasonic single-photon vertical APD pixel design

In a paper titled "Robust Pixel Design Methodologies for a Vertical Avalanche Photodiode (VAPD)-Based CMOS Image Sensor" Inoue et al. from Panasonic Japan write:

We present robust pixel design methodologies for a vertical avalanche photodiode-based CMOS image sensor, taking account of three critical practical factors: (i) “guard-ring-free” pixel isolation layout, (ii) device characteristics “insensitive” to applied voltage and temperature, and (iii) stable operation subject to intense light exposure. The “guard-ring-free” pixel design is established by resolving the tradeoff relationship between electric field concentration and pixel isolation. The effectiveness of the optimization strategy is validated both by simulation and experiment. To realize insensitivity to voltage and temperature variations, a global feedback resistor is shown to effectively suppress variations in device characteristics such as photon detection efficiency and dark count rate. An in-pixel overflow transistor is also introduced to enhance the resistance to strong illumination. The robustness of the fabricated VAPD-CIS is verified by characterization of 122 different chips and through a high-temperature and intense-light-illumination operation test with 5 chips, conducted at 125 °C for 1000 h subject to 940 nm light exposure equivalent to 10 kLux. 

 

Open access link to full paper:  https://www.mdpi.com/1424-8220/24/16/5414

Cross-sectional views of a pixel: (a) a conventional SPAD and (b) a VAPD-CIS. N-type and P-type regions are drawn by blue and red, respectively.
 

(a) A chip photograph of VAPD-CIS overlaid with circuit block diagrams. (b) A circuit diagram of the VAPD pixel array. (c) A schematic timing diagram of the pixel circuit illustrated in (b).
 

(a) An illustrative time-lapsed image of the sun. (b) Actual images of the sun taken at each time after starting the experiment. The test lasted for three hours, and as time passed, the sun, initially visible on the left edge of the screen, moved to the right.