Single-photon avalanche diode

A single-photon avalanche diode (SPAD)is a solid-state photodetector in which a photon-generated carrier (via the internal photoelectric effect) can trigger a short-duration but relatively large avalanche current. This avalanche is created through a mechanism called impact ionization, whereby carriers (electrons and/or holes) are accelerated to high kinetic energies through a large potential gradient (voltage). If the kinetic energy of a carrier is sufficient (as a function of the ionization energy of the bulk material) further carriers are liberated from the atomic lattice. The number of carriers thus increases exponentially from, in some cases, as few as a single carrier. This mechanism was observed and modeled by John Townsend for trace-gas vacuum tubes, becoming known as a Townsend discharge, and later being attributed to solid-state breakdown by K. McAfee. This device is able to detect low-intensity ionizing radiation, including: gamma, X-ray, beta, and alpha-particle radiation along with electromagnetic signals in the UV, Visible and IR (in the optical case this can be down to the single photon level). SPADs are also able to distinguish the arrival times of events (photons) with a timing jitter of a few tens of picoseconds. A single-photon avalanche diode (SPAD)is a solid-state photodetector in which a photon-generated carrier (via the internal photoelectric effect) can trigger a short-duration but relatively large avalanche current. This avalanche is created through a mechanism called impact ionization, whereby carriers (electrons and/or holes) are accelerated to high kinetic energies through a large potential gradient (voltage). If the kinetic energy of a carrier is sufficient (as a function of the ionization energy of the bulk material) further carriers are liberated from the atomic lattice. The number of carriers thus increases exponentially from, in some cases, as few as a single carrier. This mechanism was observed and modeled by John Townsend for trace-gas vacuum tubes, becoming known as a Townsend discharge, and later being attributed to solid-state breakdown by K. McAfee. This device is able to detect low-intensity ionizing radiation, including: gamma, X-ray, beta, and alpha-particle radiation along with electromagnetic signals in the UV, Visible and IR (in the optical case this can be down to the single photon level). SPADs are also able to distinguish the arrival times of events (photons) with a timing jitter of a few tens of picoseconds. SPADs, like avalanche photodiodes (APDs), exploit the incident radiation triggered avalanche current of a p–n junction when reverse biased. The fundamental difference between SPADs and APDs is that SPADs are specifically designed to operate with a reverse-bias voltage well above the breakdown voltage. This kind of operation is also called Geiger-mode in the literature (as opposed to the linear-mode for the case of an APD). This is in analogy with the Geiger counter. Since the 1970s, the applications of SPADs have increased significantly. Recent examples of their use include LIDAR, Time of Flight (ToF) 3D Imaging, PET scanning, single-photon experimentation within physics, fluorescence lifetime microscopy and optical communications (particularly quantum key distribution). Notable companies that have commercialized SPAD technology include: ST Microelectronics, Tower Jazz, Phillips and Micro Photon Devices (MPD). The related technologies of solid-state silicon photomultipliers (Si-PMs) and multi-pixel photon counters (MPPCs) have been commercialized and available through companies such as SensL (currently part of ON Semiconductor) and Hamamatsu. The history and development of SPADs and APDs shares a number of important points with the development of solid-state technologies such as diodes and early p–n junction transistors (particularly war-efforts at Bell Labs). This history can be traced to the late 1890s and early 1900s, by Prof. Brendan O Callaghan, UCC, BEng, Donoughmore et al, however suitable references for the historical development of these devices, can be found for the years 1900 to 1969 , along with a number of overview historical and technical reviews . John Townsend in 1901 and 1903 investigated the ionisation of trace gases within vacuum tubes, finding that as the electric potential increased, gaseous atoms and molecules could become ionised by the kinetic energy of free electrons accelerated though the electric field. The new liberated electrons were then themselves accelerated by the field, producing new ionisations once their kinetic energy has reached sufficient levels. This theory was later instrumental in the development of the thyratron and the Geiger-Mueller Tube. The Townsend Discharge was also instrumental as a base theory for electron multiplication phenomena, (both DC and AC), within both Silicon and Germanium . However, the major advances in early discovery and utilisation of the avalanche gain mechanism were a product of the study of Zener breakdown, related (avalanche) breakdown mechanisms and structural defects in early silicon and germanium transistor and p–n junction devices. These defects were called 'microplasmas' and are critical in the history of APDs and SPADs. Likewise investigation of the light detection properties of p–n junctions is crucial, especially the early 1940s findings by Russel Ohl. Light detection in semiconductors and solids through the internal photoelectric effect is older with Foster Nix pointing to the work of Gudden and Pohl in the 1920s. In the 1950s and 1960s, significant effort was made to reduce the number of Microplasma breakdown and noise sources, with artificial microplasmas being fabricated for study. It became clear that the avalanche mechanism could be useful for signal amplification within the diode itself, as both light and alpha particles were used for the study of these devices and breakdown mechanisms. In the early 2000s, SPADs have been implemented within CMOS processes. This has radically increased their performance, (dark count rate, jitter, array pixel pitch etc), and has leveraged the analog and digital circuits that can be implemented alongside these devices. Notable circuits include photon counting using fast digital counters, photon timing using both time-to-digital converters (TDCs) and time-to-analog converters (TACs), passive quenching circuits using either NMOS or PMOS transistors in place of poly-silicon resistors, active quenching and reset circuits for high counting rates, and many on-chip digital signal processing blocks. Such devices, now reaching optical fill factors of >70%, with >1024 SPADs, with DCRs < 10Hz and jitter values in the 50ps region are now available with dead times of 1-2ns. Recent devices have leaveraged 3D-IC technologies such as through-silicon-vias (TSVs) to present a high-fill-factor SPAD optimised top CMOS layer (90nm or 65nm node) with a dedicated signal processing and readout CMOS layer (45nm node). Significant advancements in the noise terms for SPADs have been obtained by silicon process modelling tools such as TCAD, where guard rings, junction depths and device structures and shapes can be optimised prior to validation by experimental SPAD structures. SPADs are semiconductor devices based on a p–n junction reverse-biased at a voltage Va that exceeds breakdown voltage VB of the junction (Figure 1). 'At this bias, the electric field is so high that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly to a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks the arrival time of the detected photon.' The current continues until the avalanche is quenched by lowering the bias voltage VD down to or below VB: the lower electric field is no longer able to accelerate carriers to impact-ionize with lattice atoms, therefore current ceases. In order to be able to detect another photon, the bias voltage must be raised again above breakdown.