Single photon avalanche diode

Operating principle

SPADs are semiconductor devices based on a p-n junction reverse-biased at a voltage V_a that exceeds breakdown voltage V_B of the junction (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 V_D down to or below V_B: 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:

1. sense the leading edge of the avalanche current.
2. generate a standard output pulse synchronous with the avalanche build-up.
3. quench the avalanche by lowering the bias down to the breakdown voltage.
4. restore the photodiode to the operative level.

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

Passive quenching

The simplest quenching circuit is commonly called Passive Quenching Circuit and composed of a single resistor in series to 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 R_L (about 100 kΩ or more). After the quenching of the avalanche current, the SPAD bias V_D slowly recovers to V_a, and therefore the detector is ready to be ignited again. A detailed description of the quenching process is reported by Zappa et al.

Active quenching

A more advanced quenching scheme is called active quenching. In this case a fast discriminator senses the steep onset of the avalanche current across a 50 Ω resistor and provides a digital (CMOS, TTL, ECL, NIM) output pulse, synchronous with the photon arrival time. It then quickly reduces the bias voltage to below breakdown, then relatively quickly returns bias to above the breakdown voltage ready to sense the next photon.

Photon counting and timing

The intensity of the signal is obtained by counting (photon counting) the number of output pulses within a measurement time slot, while the time-dependent waveform of the signal is obtained by measuring the time distribution of the output pulses (photon timing). The latter is obtained by means of operating the Single Photon Avalanche Diode (SPAD) detector in Time Correlated Single Photon Counting (TCSPC) mode.

Saturation

While the avalanche recovery circuit is quenching the avalanche and restoring bias, the SPAD cannot detect photons. Any photons 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 (hundred million counts per second or more). This can be harmful to the SPAD as it will be experiencing avalanche current nearly continuously.

Internal noise and afterpulsing

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 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).

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.

I-V characteristic

If a SPAD is observed by an analogue curve-tracer, it is possible to observe a bifurcation of the current-voltage characteristics beyond breakdown, during the voltage sweeps applied to the device. When the avalanche is triggered, the SPAD sustains the avalanche current (on-branch), instead when no carrier has been generated (by a photon or a thermal generation), no charge flows through the SPAD (off-branch). If the SPAD is triggered during a sweep above breakdown, a transition from the off-branch to the on-branch can be easily observed (like a "flickering").

Comparison with APDs

Both APDs and SPADs are reversely biased semiconductor p–n junctions. However, APDs are biased close to, but not exceeding the breakdown voltage of the semiconductor. This high electric field provides an internal multiplication gain only on the order of few hundreds, since the avalanche process is not diverging as in SPADs. The resulting avalanche current intensity is linearly related to the optical signal intensity. A SPAD, however, operates with a bias voltage above the breakdown voltage. Because the device is operating in this unstable above-breakdown regime, a single photon (or a single dark-current electron) can set off a significant avalanche of electrons. Practically, this means that in an APD a single photon produces only tens or few hundreds of electrons, but in a SPAD a single photon triggers a current in the milliampere region (billions of billions of electrons per second) that can be easily "counted".

Therefore, while the APD is a linear amplifier for the input optical signal with limited gain, the SPAD is a trigger device, so the gain concept is meaningless.