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This article examines a type of photodetector known as a single-photon avalanche diode (SPAD), which are often used for highly sensitive photon-capturing environments; where other detectors are not sensitive enough to perform the task.
SPADs have become the detector of choice when other photodetectors cannot recognize the difference between noise and a signal. SPADs can detect very low signal intensities (down to the single-photon level) and can determine the arrival of a single photon at the picosecond level.
SPADs are solid-state semiconductor devices and are very similar to avalanche photodiodes (APDs). These photodetectors utilize an internal photoelectric effect—the emission of an electron or another charge carrier when a material is hit with photons of light—to generate an avalanche current. This is utilized via a p-n semiconducting junction, which is reverse-biased at a higher voltage than the breakdown voltage.
This is a critical factor that differentiates SPADs from APDs, as APDs work via a reverse-biased p-n junction, but operate at a bias less than the breakdown voltage. The ability to operate above the breakdown voltage enables a higher concentration of electrons and holes to be generated.
Photodetectors, like many types of sensor, respond to detection, and this response is often based on a change in electrical conductivity in the active sensing materials. In general, the higher the electrical conductivity or charge carrier mobility, the more significant the sensitivity of the device. The ability to generate more charge carriers is what has led to SPADs being more sensitive than other photodetectors.
As the electrical voltage bias is so high (greater than 3 x 105 Vcm-1), the injection of a single charge carrier into the depletion layer of the p-n junction can cause a self-sustaining electron avalanche. The electron avalanche is generated at the atomic level through the emission of secondary electrons. These secondary electrons are released when the photon hits the active material.
The internal electric field generated by the high voltage bias causes these secondary electrons to accelerate and impact the atoms in the material’s ionic lattice, which in turn causes more atoms to release electrons. The effect is an avalanche of electrons being released through multiple impacts at the atomic level.
The impact time is not only recorded when the photon hits the active sensing material, but the avalanche causes the current to increase from a nanoscopic level to a macroscopic level and remain steady in the milliamp (mA) range.
The general mode of operation in SPADs leads to high currents, which, if not carefully monitored and controlled, can cause damage to the photodetector, causing it to be ineffective. The current is reduced by quenching the avalanche by lowering the bias voltage to at least the breakdown level, if not below it. This means that the electrons in the SPAD do not have enough energy to continue colliding with atoms, and the electronic field is not strong enough to continue accelerating the electrons.
Quenching can be carried out in a couple of ways. The first is through a passive quenching approach using a series resistor or a thermoelectric cooler. The second is more complex and requires a quenching circuit. This approach requires the leading edge of the avalanche to be sensed and identified.
An output pulse must then be generated, which is the same as the avalanche build-up. The voltage bias level is then quenched to the breakdown level or below, and this restores the SPAD to its original operating level. Regardless of the quenching mechanism applied, it enables the current to be reduced to safe levels so that the SPAD stops conducting after the detection has occurred.
The high sensitivity of SPADs lends them to applications where other photodetectors are not sensitive enough, and so they are often found in specialist applications; such as in spectroscopy instruments, LiDAR applications, DNA analysis, particle measurement instruments, fluorescence microscopes, and single molecule detection. As technologies advance, it is thought that SPADs may also play a role in quantum cryptography applications.