A single-photon avalanche diode (SPAD), also called Geiger-mode avalanche photodiode (G-APD or GM-APD) is a solid-state photodetector within the same family as photodiodes and avalanche photodiodes (APDs), while also being fundamentally linked with basic diode behaviours. As with photodiodes and APDs, a SPAD is based around a semi-conductor p-n junction that can be illuminated with ionizing radiation such as gamma, x-rays, beta and alpha particles along with a wide portion of the electromagnetic spectrum from ultraviolet (UV) through the visible wavelengths and into the infrared (IR).

In a photodiode, with a low reverse bias voltage, the leakage current changes linearly with absorption of photons, i.e. the liberation of current carriers (electrons and/or holes) due to the internal photoelectric effect. However, in a SPAD, the reverse bias is so high that a phenomenon called impact ionisation occurs which is able to cause an avalanche current to develop. Simply, a photo-generated carrier is accelerated by the electric field in the device to a kinetic energy which is enough to overcome the ionisation energy of the bulk material, knocking electrons out of an atom. A large avalanche of current carriers grows exponentially and can be triggered from as few as a single photon-initiated carrier. A SPAD is able to detect single photons providing short duration trigger pulses that can be counted. However, they can also be used to obtain the time of arrival of the incident photon due to the high speed that the avalanche builds up and the device's low timing jitter.

The fundamental difference between SPADs and APDs or photodiodes, is that a SPAD is biased well above its reverse-bias breakdown voltage and has a structure that allows operation without damage or undue noise. While an APD is able to act as a linear amplifier, the level of impact ionisation and avalanche within the SPAD has prompted researchers to liken the device to a Geiger-counter in which output pulses indicate a trigger or "click" event. The diode bias region that gives rise to this "click" type behaviour is therefore called the "Geiger-mode" region.

As with photodiodes the wavelength region in which it is most sensitive is a product of its material properties, in particular the energy bandgap within the semiconductor. Many materials including silicon, germanium,A. Tosi, A.D. Mora, F. Zappa and S. Cova, "Germanium and InGaAs/InP SPADs for single-photon detection in the near-infrared" Proc. SPIE 6771, 67710P-1 (2007) - DOI: https://doi.org/10.1117/12.734961 germanium on siliconP. Vines, K. Kuzmenko, J. Kirdoda, D.C.S. Dumas, M.M. Mirza, R.W. Millar, D.J. Paul and G.S. Buller "High performance planar germanium-on-silicon single-photon avalanche diode detectors" Nature Communications 10, 1086 (2019) - DOI: https://doi.org/10.1038/s41467-019-08830-w and III-V elements such as InGaAs/InPS. Pellegrini, R.E. Warburton, L.J.J. Tan, J.S. Ng, A.B. Krysa, K. Groom, J.P.R. David, S. Cova, M.J. Robertson and G. S. Buller, "Design and performance of an InGaAs-InP single-photon avalanche diode detector" IEEE J. Quantum Elec. 42(4), pp 397 - 403 (2006) - DOI: https://doi.org/10.1109/JQE.2006.871067 have been used to fabricate SPADs for the large variety of applications that now utilise the run-away avalanche process. There is much research in this topic with activity implementing SPAD-based systems in CMOS fabrication technologies, and investigation and use of III-V material combinations and Ge on Si L. Ferre Llin, J. Kirdoda, F. Thorburn, L.L. Huddleston, Z.M. Greener, K. Kuzmenko, P. Vines, D.C.S. Dumas, R.W. Millar, G.S. Buller and D.J. Paul, "High sensitivity Ge-on-Si single-photon avalanche diode detectors" Optics Letters 45(23), pp. 6406 - 6409 (2020) - DOI: https://doi.org/10.1364/OL.396756 for single-photon detection at short-wave infrared wavelengths suitable for telecommunications applications.

Applications 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, optical communications (particularly quantum key distribution), digital night vision, and ultrahigh sensitivity cameras.

Operation

Structures SPADs are semiconductor devices that are based on a p–n junction that is reverse-biased at an operating voltage that exceeds the junction's breakdown voltage (Figure 1). "At this bias, the electric field is so high [higher than 3×105 V/cm] that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly [sub-nanosecond rise-time] to a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks [with picosecond time jitter] the arrival time of the detected photon." The current continues until the avalanche is quenched by lowering the bias voltage down to or below the breakdown voltage: 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.

"This operation requires a suitable circuit, which has to: Sense the leading edge of the avalanche current. Generate a standard output pulse synchronous with the avalanche build-up. Quench the avalanche by lowering the bias down to the breakdown voltage. Restore the photodiode to the operative level.

This circuit is usually referred to as a quenching circuit."

Biasing regions and current-voltage characteristic

A semiconductor p-n junction can be biased at several operating regions depending on the applied voltage. For normal uni-directional diode operation, the forward biasing region and the forward voltage are used during conduction, while the reverse bias region prevents conduction. When operated with a low reverse bias voltage, the p-n junction can operate as a unity gain photodiode. As the reverse bias increases, some internal gain through carrier multiplication can occur allowing the photodiode to operate as an avalanche photodiode (APD) with a stable gain and a linear response to the optical input signal. However, as the bias voltage continues to increase, the p-n junction breaks down when the electric field strength across the p-n junction reaches a critical level. As this electric field is induced by the bias voltage over the junction it is denoted as the breakdown voltage, VBD. A SPAD is reverse biased with an excess bias voltage, Vex, above the breakdown voltage, but below a second, higher breakdown voltage associated with the SPAD's guard ring. The total bias (VBD+Vex) therefore exceeds the breakdown voltage to such a degree that "At this bias, the electric field is so high [higher than 3×105 V/cm] that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly [sub-nanosecond rise-time] to a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks [with picosecond time jitter] the arrival time of the detected photon".

As the current vs voltage (I-V) characteristic of a p-n junction gives information about the conduction behaviour of the diode, this is often measured using an analogue curve-tracer. This sweeps the bias voltage in fine steps under tightly controlled laboratory conditions. For a SPAD, without photon arrivals or thermally generated carriers, the I-V characteristic is similar to the reverse characteristic of a standard semi-conductor diode, i.e. an almost total blockage of charge flow (current) over the junction other than a small leakage current (nano-amperes). This condition can be described as an "off-branch" of the characteristic.

However, when this experiment is conducted, a "flickering" effect and a second I-V characteristic can be observed beyond breakdown. This occurs when the SPAD has experienced a triggering event (photon arrival or thermally generated carrier) during the voltage sweeps that are applied to the device. The SPAD, during these sweeps, sustains an avalanche current which is described as the "on-branch" of the I-V characteristic. As the curve tracer increases the magnitude of the bias voltage over time, there are times that the SPAD is triggered during the voltage sweep above breakdown. In this case a transition occurs from the off-branch to the on-branch, with an appreciable current starting to flow. This leads to the flickering of the I-V characteristic that is observed and was denoted by early researchers in the field as "bifurcation" (def: the division of something into two branches or parts). To detect single-photons successfully, the p-n junction must have very low levels of the internal generation and recombination processes. To reduce thermal generation, devices are often cooled, while phenomena such as tunnelling across the p-n junctions also need to be reduced through careful design of semi-conductor dopants and implant steps. Finally, to reduce noise mechanisms being exacerbated by trapping centres within the p-n junction's band gap structure the diode needs to have a "clean" process free of erroneous dopants.

Passive quenching circuits The simplest quenching circuit is commonly called passive quenching circuit and comprises a single resistor in series with the SPAD. This experimental setup has been employed since the early studies on the avalanche breakdown in junctions. The avalanche current self-quenches simply because it develops a voltage drop across a high-value ballast load RL (about 100 kΩ or more). After the quenching of the avalanche current, the SPAD bias slowly recovers to the operating bias, and therefore the detector is ready to be ignited again. This circuit mode is therefore called passive quenching passive reset (PQPR), although an active circuit element can be used for reset forming a passive quench active reset (PQAR) circuit mode. A detailed description of the quenching process is reported by Zappa et al.

Active quenching circuits A more advanced quenching, which was explored from the 1970s onwards, is a scheme called active quenching. In this case a fast discriminator senses the steep onset of the avalanche current across a 50 Ω resistor (or integrated transistor) and provides a digital (CMOS, TTL, ECL, NIM) output pulse, synchronous with the photon arrival time. The circuit then quickly reduces the bias voltage to below breakdown (active quenching), then relatively quickly returns bias to above the breakdown voltage ready to sense the next photon. This mode is called active quench active reset (AQAR), however depending on circuit requirements, active quenching passive reset (AQPR) may be more suitable. AQAR circuits often allow lower dead times, and significantly reduced dead time variation.

Photon counting and saturation