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A single-photon avalanche diode (SPAD) (also known as a Geiger-mode APD or G-APD) is a solid-state photodetector in which a photon-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 avalanche photodiodes (APDs), exploit 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 SPADs are specifically designed to operate with a reverse-bias voltage well above the breakdown voltage. This kind of operation is also called ''Geiger mode'' in literature, in analogy with the Geiger counter. == Operating principle == SPADs are semiconductor devices based on a p-n junction reverse-biased at a voltage Va that exceeds breakdown voltage VB of the junction (). "At this bias, the electric field is so high (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 (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 (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.〔 "This operation requires a suitable circuit, which has to: # sense the leading edge of the avalanche current. # generate a standard output pulse synchronous with the avalanche build-up. # quench the avalanche by lowering the bias down to the breakdown voltage. # restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit."〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Single-photon avalanche diode」の詳細全文を読む スポンサード リンク
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