# Current-Assisted Single Photon Avalanche Diode (CASPAD) Fabricated in 350 nm Conventional CMOS
## 1 Introduction  
Single-photon avalanche diodes (SPADs) become an integral part of applications which require high photon sensitivity, and/or accurate photon arrival timing. SPAD versions based on complementary metal-oxide semiconductor (CMOS) technology have the advantage of low production cost and the ability to monolithically integrate them with electronic circuitry for read-out and signal processing. They are employed in various applications such as 3D time-of-flight (TOF) and fluorescence-lifetime imaging (FLI) [1; 2; 3; 4; 5].  
A SPAD is a p-n junction biased above its reverse breakdown voltage. At this bias, the SPAD operates in a meta-stable Geiger regime where a single electron/hole can generate impact ionization and the resulting electrons/holes also generate impact ionization and so on and therefore a large reverse current flows, which is self-sustaining. Therefore, the gain for this mode of operation is virtually infinite. The reverse current needs to be quenched to protect the p-n junction from overheating and physical damage. Quenching is also necessary to restore the SPAD to detect further photons. Quenching [6] is done by using a load resistor (passive quenching) or an active circuit of transistors (active quenching). In a conventional SPAD, the incoming light generates photo-carriers inside or near the depletion region of a horizontal p-n junction where the multiplication is expected to happen. Photo-carriers that are generated farther from the depletion region slowly diffuse and can potentially also cause avalanche breakdown. Increasing the detection area is not very straight-forward because that increases the active area where the multiplication is expected, and this puts more emphasis on careful design to have uniform field all over the active area. Increasing the detection area also increases the diode capacitance which in turn leads to a larger dead time of the detector. Another important concern isthat a larger detection area increases the number of traps in the active area where the multiplication occurs and thereby increasing the probability of afterpulsing [7]. Further, sensitivity in near infrared (NIR) wavelengths, so important to many applications, is also posing problems, because the associated large photon penetration (10 \(\upmu\)m-20 \(\upmu\)m) depth is difficult to match with the limited thickness of the horizontal depletion region of the avalanching junction (several microns). Retrograde wells with variable doping profile for generating a built-in electric field have been introduced improving the NIR sensitivity of SPADs [8].  
In this work, we present the current-assisted single-photon avalanche diode (CASPAD) that has a small p-n junction located in the center for avalanche breakdown and a large detection volume around it for photon absorption. The photo-electrons in the detection volume are guided using a drift field toward the central \"SPAD\" where they initiate an avalanche breakdown and can be subsequently detected by attached circuitry. Separate absorption and multiplication region avalanche photodiodes (SAM APDs [9]) in III-V semiconductors exist also having infrared sensitivity, but these are not in CMOS and thus require 3D hybrid stacking for read-out and signal-processing circuitry.