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Fundamentals of CMOS image sensors
Published in Jun Ohta, Smart CMOS Image Sensors and Applications, 2020
In these reports, the APD is biased over the avalanche breakdown voltage, and thus when the photons are incident on the APD, it quickly turns on. As shown in Fig. 2.12, by being connected serially with a quench resistor, it can be turned off and thus, produces a spike-like current pulse as shown in Fig. 2.12. It can detect a single photon and hence, it is called single photon avalanche diode (SPAD). The initial position of SPAD is shown in (1) in Fig. 2.12 (b). When a photon hits on a SPAD, it quickly turns on ((1) → (2)) because it is biased well above the avalanche breakdown voltage, Vo in Fig. 2.12 (b), and a photocurrent flows. This photocurrent drops the bias voltage to the SPAD by means of the quench resistor Rq. Thus, the applied voltage to the SPAD goes under the avalanche breakdown voltage, and finally stops the breakdown ((2) → (3)). After quenching, the SPAD is charged again and recovered at the initial point (1). This phenomenon resembles that of a Geiger counter and thus it is called the Geiger mode.
Theory of photon detectors
Published in Antoni Rogalski, Infrared and Terahertz Detectors, 2019
It should be mentioned that the APD can also be biased at voltage larger than the infinity-gain voltage in such a way that the arrival of a single photon precipitates avalanche breakdown, thereby creating a large current pulse that signifies a subsequent photon. This may be carried out either by passive or active means. This working regime is called counting mode or single photon avalanche detector (SPAD), also known as a Geiger-mode avalanche detector, after the work pioneered by Cova and coworkers [122]. SPAD is potentially very sensitive, comparable to that of photomultipliers. However, it has to be noted that once the avalanche at infinite gain is initiated, further photons eventually detected during pulse duration and circuit recovery time are ignored. From this point of view, the SPAD is similar to a Geiger counter than a photomultiplier.
CMOS Implementation of Time Delay Integration (TDI) for Imaging Applications: A Brief Review
Published in IETE Technical Review, 2020
Sushil Kumar Semwal, Raghvendra Sahai Saxena
Kong et al. proposed time delay integration in single photon avalanche diode (SPAD) [44]. In SPAD one output pulse represents one photon. Thus the signal intensity is defined by the total number of pulses at output. Figure 25 shows the schematic block diagram of Multi-stage TDI-SPAD.