Detectors 188
Detectors 188 : Particle Detectors: (4.25) Single-Photon Avalanche Diode: (1) Operating Principle: SPADs are semiconductor devices based on a p-n junction reverse-biased at a voltage Va higher than VB (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 VD down to or below VB : 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. Figure 1 - Thin SPAD cross-section". This operation requires a suitable circuit, which has to: (a) sense the leading edge of the avalanche current; (b) generate a standard output pulse synchronous with the avalanche build-up; (c) quench the avalanche by lowering the bias down to the breakdown voltage; (d) restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit"
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