Single-photon avalanche diode
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).
However, in a SPAD,[3][4] the reverse bias is so high that a phenomenon called impact ionisation occurs which is able to cause an avalanche current to develop.
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.
There is much research in this topic with activity implementing SPAD-based systems in CMOS fabrication technologies,[8] and investigation and use of III-V material combinations[9] and Ge on Si [10] for single-photon detection at short-wave infrared wavelengths suitable for telecommunications applications.
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).
[3] "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.
[3] 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 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.
This leads to the flickering of the I-V characteristic that is observed and was denoted by early researchers in the field as "bifurcation"[4] (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.
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.
The avalanche current self-quenches simply because it develops a voltage drop across a high-value ballast load RL (about 100 kΩ or more).
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.
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.
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.
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.
This method is useful for 3D imaging, LIDAR and is used heavily in physical measurements relying on time-correlated single photon counting (TCSPC).
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.
[21][22] 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.
[19] 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.
[29] 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 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.
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.
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.
The Townsend discharge was also instrumental as a base theory for electron multiplication phenomena, (both DC and AC), within both silicon and germanium.
[citation needed] 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.
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.
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.
