Single-photon Avalanche Diode
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A single-photon avalanche diode (SPAD) is a solid-state
photodetector Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...
within the same family as
photodiode A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
s and
avalanche photodiode An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of phot ...
s (APDs), while also being fundamentally linked with basic
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A 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 Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
such as gamma, x-rays, beta and alpha particles along with a wide portion of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
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 The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
. 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 An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
in the device to a
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
which is enough to overcome the
ionisation energy Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
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 In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
. 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 A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
. Many materials including
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
,
germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
and other
III-V Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of ...
elements 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 Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
fabrication technologies, and investigation and use of III-V material combinations for single-photon detection at dedicated wavelengths.


Applications

Since the 1970s, the applications of SPADs have increased significantly. Recent examples of their use include
lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
s,
time of flight Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
(ToF) 3D imaging, PET scanning, single-photon experimentation within physics, fluorescence lifetime microscopy and optical communications (particularly
quantum key distribution Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which can then be ...
).


Operation


Structures

SPADs are
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
devices based on a
p–n junction A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...
reverse-biased at an operating voltage that exceeds the junctions breakdown voltage (
Figure 1 Figure 1 is a Toronto, Ontario-based online social networking service for healthcare professionals to post and comment on medical images. Figure 1 was founded in Toronto by Dr. Joshua Landy, Richard Penner and Gregory Levey. The platform launched ...
). "At this bias, the
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
is so high igher than 3×105 V/cmthat a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly ub-nanosecond rise-timeto a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks ith picosecond time jitter the arrival time of the detected
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
." The current continues until the avalanche is quenched by lowering the
bias voltage In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...
down to or below the breakdown voltage: the lower electric field is no longer able to accelerate carriers to impact-ionize with
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
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 A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
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 A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A 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 A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
. 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 An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
is so high igher than 3×105 V/cmthat a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly ub-nanosecond rise-timeto a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks ith picosecond time jitterthe arrival time of the detected
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
". 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 Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
,
TTL TTL may refer to: Photography * Through-the-lens metering, a camera feature * Zenit TTL, an SLR film camera named for its TTL metering capability Technology * Time to live, a computer data lifespan-limiting mechanism * Transistor–transistor lo ...
, 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

The intensity of the input signal can be obtained by counting (
photon counting Photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). A single-photon detector emits a pulse of signal for each detected photon, in contrast to a normal photodetector, which generates an analo ...
) the number of output pulses within a measurement time period. This is useful for applications such as low light imaging, PET scanning and fluorescence lifetime microscopy. However, while the avalanche recovery circuit is quenching the avalanche and restoring bias, the SPAD cannot detect further photon arrivals. Any photons, (or dark counts or after-pulses), that reach the detector during this brief period are not counted. As the number of photons increases such that the (statistical) time interval between photons gets within a factor of ten or so of the avalanche recovery time, missing counts become statistically significant and the count rate begins to depart from a linear relationship with detected light level. At this point the SPAD begins to saturate. If the light level were to increase further, ultimately to the point where the SPAD immediately avalanches the moment the avalanche recovery circuit restores bias, the count rate reaches a maximum defined purely by the avalanche recovery time in the case of active quenching (hundred million counts per second or moreEisele, A.; Henderson, R.; Schmidtke, B.; Funk, T.; Grant, L.; Richardson, J.; Freude, W.
''185 MHz count rate, 139 dB dynamic range single-photon avalanche diode with active quenching circuit in 130 nm CMOS technology''
Intern. Image Sensor Workshop (IISW'11), Hokkaido, Japan; Paper R43; June 2011
). This can be harmful to the SPAD as it will be experiencing avalanche current nearly continuously. In the passive case, saturation may lead to the count rate decreasing once the maximum is reached. This is called paralysis, whereby a photon arriving as the SPAD is passively recharging, has a lower detection probability, but can extend the dead time. It is worth noting that passive quenching, while simpler to implement in terms of circuitry, incurs a 1/e reduction in maximum counting rates.


Dark count rate (DCR)

Besides photon-generated carriers, thermally-generated carriers (through generation-recombination processes within the semiconductor) can also fire the avalanche process. Therefore, it is possible to observe output pulses when the SPAD is in complete darkness. The resulting average number of counts per second is called ''dark count rate'' (DCR) and is the key parameter in defining the detector noise. It is worth noting that the reciprocal of the dark count rate defines the mean time that the SPAD remains biased above breakdown before being triggered by an undesired thermal generation. Therefore, in order to work as a single-photon detector, the SPAD must be able to remain biased above breakdown for a sufficiently long time (e.g., a few milliseconds, corresponding to a count rate well under a thousand counts per second, cps).


Afterpulsing noise

One other effect that can trigger an avalanche is known as afterpulsing. When an avalanche occurs, the PN junction is flooded with charge carriers and trap levels between the valence and conduction band become occupied to a degree that is much greater than that expected in a thermal-equilibrium distribution of charge carriers. After the SPAD has been quenched, there is some probability that a charge carrier in a trap level receives enough energy to free it from the trap and promote it to the conduction band, which triggers a new avalanche. Thus, depending on the quality of the process and exact layers and implants that were used to fabricate the SPAD, a significant number of extra pulses can be developed from a single originating thermal or photo-generation event. The degree of afterpulsing can be quantified by measuring the autocorrelation of the times of arrival between avalanches when a dark count measurement is set up. Thermal generation produces Poissonian statistics with an impulse function autocorrelation, and afterpulsing produces non-Poissonian statistics.


Photon timing and jitter

The leading edge of a SPAD's avalanche breakdown is particularly useful for timing the arrival of photons. This method is useful for 3D imaging, LIDAR and is used heavily in physical measurements relying on
time-correlated single photon counting Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales ( attoseconds to nanoseconds). Different methods are used to examine the dynamics of charge carrie ...
(TCSPC). However, to enable such functionality dedicated circuits such as time-to-digital converters (TDCs) and time-to-analogue (TAC) circuits are required. The measurement of a photon's arrival is complicated by two general processes. The first is the statistical fluctuation in the arrival time of the photon itself, which is a fundamental property of light. The second is the statistical variation in the detection mechanism within the SPAD due to a) depth of photon absorption, b) diffusion time to the active p-n junction, c) the build up statistics of the avalanche and d) the jitter of the detection and timing circuitry.


Optical fill factor

For a single SPAD, the ratio of its optically sensitive area, Aact, to its total area, Atot, is called the fill factor, . As SPADs require a guard ring to prevent premature edge breakdown, the optical fill factor becomes a product of the diode shape and size with relation the its guard ring. If the active area is large and the outer guard ring is thin, the device will have a high fill factor. With a single device, the most efficient method to ensure full utilisation of the area and maximum sensitivity is to focus the incoming optical signal to be within the device's active area, i.e. all incident photons are absorbed within the planar area of the p-n junction such that any photon within this area can trigger an avalanche. Fill factor is more applicable when we consider arrays of SPAD devices. Here the diode active area may be small or commensurate with the guard ring's area. Likewise, the fabrication process of the SPAD array may put constraints on the separation of one guard ring to another, i.e. the minimum separation of SPADs. This leads to the situation where the area of the array becomes dominated by guard ring and separation regions rather than optically receptive p-n junctions. The fill factor is made worse when circuitry must be included within the array as this adds further separation between optically receptive regions. One method to mitigate this issue is to increase the active area of each SPAD in the array such that guard rings and separation are no longer dominant, however for CMOS integrated SPADs the erroneous detections caused by dark counts increases as the diode size increases.


Geometric improvements

One of the first methods to increase fill factors in arrays of circular SPADs was to offset the alignment of alternate rows such that the curve of one SPAD partially uses the area between the two SPADs on an adjacent row. This was effective but complicated the routing and layout of the array. To address fill factor limitations within SPAD arrays formed of circular SPADs, other shapes are utilised as these are known to have higher maximum area values within a typically square pixel area and have higher packing ratios. A square SPAD within a square pixel achieves the highest fill factor, however the sharp corners of this geometry are known to cause premature breakdown of the device, despite a guard ring and consequently produce SPADs with high dark count rates. To compromise, square SPADs with sufficiently rounded corners have been fabricated. These are termed
Fermat Pierre de Fermat (; between 31 October and 6 December 1607 – 12 January 1665) was a French mathematician who is given credit for early developments that led to infinitesimal calculus, including his technique of adequality. In particular, he i ...
shaped SPADs while the shape itself is a super-ellipse or a Lamé curve. This nomenclature is common in the SPAD literature, however the Fermat curve refers to a special case of the super-ellipse that puts restrictions on the ratio of the shape's length, "a" and width, "b" (they must be the same, a = b = 1) and restricts the degree of the curve "n" to be even integers (2, 4, 6, 8 etc). The degree "n" controls the curvature of the shape's corners. Ideally, to optimise the shape of the diode for both low noise and a high fill factor, the shape's parameters should be free of these restrictions. To minimise the spacing between SPAD active areas, researchers have removed all active circuitry from the arrays and have also explored the use of NMOS only CMOS SPAD arrays to remove SPAD guard ring to PMOS n-well spacing rules. This is of benefit but is limited by routing distances and congestion into the centre SPADs for larger arrays. The concept has been extended to develop arrays that use clusters of SPADs in so-called mini-SiPM arrangements whereby a smaller array is provided with its active circuitry at one edge, allowing a second small array to be abutted on a different edge. This reduced the routing difficulties by keeping the number of diodes in the cluster manageable and creating the required number of SPADs in total from collections of those clusters. A significant jump in fill factor and array pixel pitch was achieved by sharing the deep n-well of the SPADs in CMOS processes, and more recently also sharing portions of the guard-ring structure. This removed one of the major guard-ring to guard-ring separation rules and allowed the fill-factor to increase towards 60 or 70%. The n-well and guard ring sharing idea has been crucial in efforts towards lowering pixel pitch and increasing the total number of diodes in the array. Recently SPAD pitches have been reduced to 3.0 um and 2.2 um. Porting a concept from photodiodes and APDs, researchers have also investigated the use of drift electric fields within the CMOS substrate to attract photo generated carriers towards a SPAD's active p-n junction. By doing so a large optical collection area can be achieved with a smaller SPAD region. Another concept ported from CMOS image sensor technologies, is the exploration of stacked p-n junctions similar to
Foveon Foveon, Inc., is an American company that manufactures and distributes image sensor technology. It makes the Foveon X3 sensor, which captures images in some digital cameras. Foveon was founded in 1997 and is based in Santa Clara, California. In 2 ...
sensors. The idea being that higher-energy photons (blue) tend to be absorbed at a short absorption depth, i.e. near the silicon surface. Red and infra-red photons (lower energy) travel deeper into the silicon. If there is a junction at that depth, red and IR sensitivity can be improved.


IC fabrication improvements

With the advancement of 3D IC technologies, i.e. stacking of integrated circuits, the fill factor could be enhanced further by allowing the top die to be optimised for a high fill-factor SPAD array, and the lower die for readout circuits and signal processing. As small dimension, high-speed processes for transistors may require different optimisations than optically sensitive diodes, 3D-ICs allow the layers to be separately optimised.


Pixel-level optical improvements

As with CMOS image sensors micro-lenses can be fabricated on the SPAD pixel array to focus light into the centre of the SPAD. As with a single SPAD, this allows light to only hit the sensitive regions and avoid both the guard ring and any routing that is needed within the array. This has also recently included Fresnel type lenses.


Pixel pitch

The above fill-factor enhancement methods, mostly concentrating on SPAD geometry along with other advancements, have led SPAD arrays to recently push the 1 mega pixel barrier. While this lags CMOS image sensors (with pitches now below 0.8 um), this is a product of both the youth of the research field (with CMOS SPADs introduced in 2003) and the complications of high voltages, avalanche multiplication within the silicon and the required spacing rules.


Comparison with APDs

While both APDs and SPADs are semiconductor p-n junctions that are heavily reverse biased, the principle difference in their properties is derived from their different biasing points upon the reverse I-V characteristic, i.e. the reverse voltage applied to their junction. An APD, in comparison to a SPAD, is not biased above its breakdown voltage. This is because the multiplication of charge carriers is known to occur prior to the breakdown of the device with this being utilised to achieve a stable gain that varies with the applied voltage. For optical detection applications, the resulting avalanche and subsequent current in its biasing circuit is linearly related to the optical signal intensity. The APD is therefore useful to achieve moderate up-front amplification of low-intensity optical signals but is often combined with a trans-impedance amplifier (TIA) as the APD's output is a current rather than the voltage of a typical amplifier. The resultant signal is a non-distorted, amplified version of the input, allowing for the measurement of complex processes that modulate the amplitude of the incident light. The internal multiplication gain factors for APDs vary by application, however typical values are of the order of few hundreds. The avalanche of carriers is not divergent in this operating region, while the avalanche present in SPADs quickly builds into a run-away (divergent) condition. In comparison, SPADs operate at a bias voltage above the breakdown voltage. This is such a highly unstable above-breakdown regime that a single photon or a single dark-current electron can trigger a significant avalanche of carriers. The semiconductor p-n junction breaks down completely, and a significant current is developed. A single photon can trigger a current spike equivalent to billions of billions of electrons per second (with this being dependent on the physical size of the device and its bias voltage). This allows subsequent electronic circuits to easily count such trigger events. As the device produces a trigger event, the concept of gain is not strictly compatible. However, as the photon detection efficiency (PDE) of SPADs varies with the reverse bias voltage, gain, in a general conceptual sense can be used to distinguish devices that are heavily biased and therefore highly sensitive in comparison to lightly biased and therefore of lower sensitivity. While APDs can amplify an input signal preserving any changes in amplitude, SPADs distort the signal into a series of trigger or pulse events. The output can still be treated as proportional to the input signal intensity, however it is now transformed into the frequency of trigger events, i.e. pulse frequency modulation (PFM). Pulses can be counted giving an indication of the input signal's optical intensity, while pulses can trigger timing circuits to provide accurate time-of-arrival measurements. One crucial issue present in APDs is multiplication noise induced by the statistical variation of the avalanche multiplication process. This leads to a corresponding noise factor on the output amplified photo current. Statistical variation in the avalanche is also present in SPAD devices, however due to the runaway process it is often manifest as timing jitter on the detection event. Along with their bias region, there are also structural differences between APDs and SPADs, principally due to the increased reverse bias voltages required and the need for SPADs to have a long quiescent period between noise trigger events to be suitable for the single-photon level signals to be measured.


History, development and early pioneers

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). 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 A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, pro ...
and the Geiger-Mueller tube. The
Townsend discharge The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and ...
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 In electronics, the Zener effect (employed most notably in the appropriately named Zener diode) is a type of electrical breakdown, discovered by Clarence Melvin Zener. It occurs in a reverse biased p-n diode when the electric field enables tunn ...
, related ( avalanche) breakdown mechanisms and structural defects in early silicon and germanium transistor and p–n junction devices. These defects were called '
microplasma A microplasma is a plasma of small dimensions, ranging from tens to thousands of micrometers. Microplasmas can be generated at a variety of temperatures and pressures, existing as either thermal or non-thermal plasmas. Non-thermal microplasmas that ...
s' 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 of 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, who use the phrase primary and secondary to distinguish the internal and external photoelectric effects respectively. 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 Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
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 converter In electronic instrumentation and signal processing, a time-to-digital converter (TDC) is a device for recognizing events and providing a digital representation of the time they occurred. For example, a TDC might output the time of arrival for ea ...
s (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 < 10 Hz 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 (90 nm or 65 nm node) with a dedicated signal processing and readout CMOS layer (45 nm 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.


See also

*
Avalanche photodiode An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of phot ...
(APD) *
p–n junction A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...
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Silicon photomultiplier Silicon photomultipliers, often called "SiPM" in the literature, are solid-state single-photon-sensitive devices based on Single-photon avalanche diode (SPAD) implemented on common silicon substrate. The dimension of each single SPAD can vary fro ...
(SiPM) *
Oversampled binary image sensor An oversampled binary image sensor is an image sensor with non-linear response capabilities reminiscent of traditional photographic film. Each pixel in the sensor has a binary response, giving only a one-bit quantized measurement of the local ligh ...


References

{{Reflist Optical devices Optical diodes Particle detectors Photodetectors