Single-photon avalanche diode

In optoelectronics the term Single-Photon Avalanche Diode (SPAD) (also know as a Geiger-mode APD or G-APD) identifies a class of solid-state photodetectors based on a reverse biased p-n junction in which a photo-generated carrier can trigger an avalanche current due to the impact ionization mechanism. This device is able to detect low intensity signals (down to the single photon) and to signal the arrival times of the photons with a jitter of a few tens of picoseconds.

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.

Figure 1 - Thin SPAD cross-section.

These operations require 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 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

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

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 a Time Correlated Single Photon Counting (TCSPC) instrumentation.

Internal noise

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).

I-V Characteristic

Figure 2: Current-voltage characteristic of a SPAD showing the off- and on-branch.

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 versus SPADs

Both APDs and SPADS are reverse biased semiconductor p-n junctions. However, APDs are biased close to, but below 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 photons. 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 mA 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.

See also

• APD
• Sipm
• P-n junction

External Links

Commercially available single-photon avalanche diodes:

• PerkinElmer SPAD catalog
• id Quantique <- Have a model with dark currents as low as ~1Hz
• SensL SPAD module
• Microphoton Devices