Single-Photon Avalanche Diode

In optoelectronics the term Single-Photon Avalanche Diode (SPAD) defines a class of photodetectors able to detect low intensity signals (down to the single photon) and to signal the time of the photon arrival with high temporal resolution (few tens of picoseconds).

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SPADs, like the Avalanche photodiode (APD), exploits the photon-triggered avalanche current of a reverse biased p-n junction to detect an incident radiation. The fundamental difference between SPAD and APD is that SPAD are specifically designed to operate with a reverse bias voltage well above the breakdown voltage (on the contrary APD operate at a bias lesser than the breakdown voltage). This kind of operation is also called Geiger mode in literature, for the analogy with the Geiger counter.

SPAD operating principle

SPADs are semiconductor devices based on a p-n junction reversed biased at a voltage V_a higher than V_B (Figure 1). At this bias, the electric field is so high (higher than 3x10⁵V/cm) that a single charge carrier injected in 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 to flow until the avalanche is quenched by lowering the bias voltage V_D down to or below V_B: the lower electric field is not able any more to accelerate the 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.

These operations require a suitable circuit, which has to i) sense the leading edge of the avalanche current; ii) generate a standard output pulse synchronous with the avalanche build-up; iii) quench the avalanche by lowering the bias down to the breakdown voltage; iv) restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit.

The simplest quenching circuit is commonly called Passive Quenching Circuit and composed of a single resistor in series to the SPAD. This experimental set-up 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 100kΩ 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 in [1].

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 50Ω resistor and provides a digital (CMOS, TTL, ECL, NIM) output pulse, synchronous with the photon arrival time.

The intensity of the signal is obtained by counting (photon counting) the number of output pulses within a measurement time slot, while the time-depended waveform of the signal is obtained by measuring the time distribution of the output pulses (photon timing). The latter is obtained by means of a Time Correlated Single Photon Counting (TCSPC) instrumentation.

Besides photon-generated carriers, also thermally-generated carriers (through generation-recombination processes within the semiconductor) can fire an avalanche process. Therefore, it is possible to observe output pulses also when the SPAD is kept in dark: the resulting average number of ignitions 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., longer than few milliseconds, corresponding to a count rate of few kilo counts per second, kcps).

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 current 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").

APDs vs. SPADs

Let us compare SPADs with APDs (Avalanche Photodiodes). Also APDs are reverse biased semiconductor p-n junctions. However, they are biased close, but below the breakdown voltage, where the high electric field provides an internal multiplication gain (of the order of tens or few hundreds), though the avalanche process is not diverging as in SPADs. The resulting avalanche current intensity is linearly related to the optical signal intensity. Therefore, while in an APD a single photon produces only tens or few hundreds of electrons, in a SPAD a single photon triggers a current in the mA region (billions of billions of electrons per second) that can be easily "counted".

Figure 4 - APDs are biased just below V_B in order to have a non-diverging multiplication process, thus producing a linear multiplication of the carriers photo-generated (left side). On the contrary SPAD are biased well above V_B and even one photo-generated carrier can trigger a diverging avalanche multiplication process leading to a macroscopic detectable output current (right side).

References

[1]

S. Cova, M. Ghioni, A.L. Lacaita, C. Samori and F. Zappa, Avalanche photodiodes and quenching circuits for single-photon detection, Applied Optics, vol. 35, no. 12, pp. 1956-1976, 1996.