Single Photon Avalanche Photodiodes

PMTs use the emission of photoelectrons from a photocathode, i.e. the extemal photo effect , as a primary step of detection. The drawback of the external photo effect is that the photoelectrons are emitted in all directions, including back into the photocathode. Therefore the quantum efficiency, i.e. the probability that a photon releases an photoelectron, is smaller than 0.5. The best cathodes reach a quantum efficiency of about 0.4 between 400 and 500 nm [214]. 

Semiconductor detectors use the intemal photo effect . That means that the photons generate electron-hole pairs inside the semiconductor. Theoretically the internal photoeffect works with a quantum efficiency of 1. In practice the quantum efficiency of a good silicon photodiode reaches 0.8 around 800 nm. In photodiodes and photoconductors an electrical field separates the electrons and holes, so that a photocurrent flows through the device when it is illuminated. Of course, the photocurrent caused by a single electron-hole pair is far too small to be recorded directly. Single photons can therefore be detected only if the semiconductor detec- 

APDs suitable for single photon detection must be free of premature breakdown at the edge of the junction or at local lattice defects. So far, only selected silicon APDs can be operated in the passive or active quenching mode, and only a few single photon APD detectors are commercially available [245, 354, 408]. 

For detection in the infrared region the situation is even less favourable. A few TCSPC applications of liquid-nitrogen cooled Ge APDs have been reported [391], but have not resulted in commercially manufactured detectors. InGaAs APDs suffer from strong afterpulsing which has prevented continuous quenched operation so far. 

Passively and actively quenched single-photon APDs must normally be cooled. Except for diodes of extremely small area the thermal carrier generation rate at 

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Figure 12.25. Wavelength dependence in the temporal response of a single-photon avalanche photodiode.

Single photon Avalanche Photodiode. An avalanche photodiode (APD) is operated above the breakdown voltage. A detected photon causes an avalanche breakdown with an easily detected current pulse. SPAD operation requires an APD with uniform break-... 

Single photon Avalanche Photodiode, see SPAD. Time-to-Amplitude Converter. Converts the time between a start and a stop pulse into a voltage. TACs can be built with a resolution down to a few picoseconds. 

Like all photon counting techniques, gated photon counting uses a fast, high-gain detector, which is usually a PMT or a single-photon avalanche photodiode. Due to the moderate time resolution of the gating technique, there are no special requirements to the transit time spread of the detector. However, the transit time distribution should be free of bumps, prepulses or afterpulses, and should remain stable up to a count rate of several tens of MHz. 

Currently available single photon avalanche photodiodes (SPADs) are not applicable to optical tomography. Although the efficiency in the NIR can be up to 80%, the detector area is only of the order of 0.01 mm. Diffusely emitted light cannot be concentrated on such a small area. A simple calculation shows that SPADs carmot compete with PMTs unless their active area is increased considerably. Another obstacle is the large IRF count-rate dependence sometimes found in single-photon APDs. 

Single photon avalanche photodiodes (SPADs) achieve the highest radiant sensitivity of all detectors in the NIR. Currently available APD detectors have ex-... 

Recently new single-photon avalanche photodiodes have been introduced, see Sect. 6.4.10 page 258. Compared with the SPCM-AQR the new devices have a considerably improved timing behaviour but lower quantum efficiency in the NIR. However, the efficiency below 600 nm is comparable or even better than for the SPCM-AQR. It is likely though not proved that these detectors are superior to the SPCM-AQR for correlation measurements in the visible spectral range. 

Fig. 6.8 Single photon avalanche photodiode (SPAD). Left Passive quenching, right active quenching...

TTS exists also in single photon avalanche photodiodes (SPADs). The source of TTS in SPADs is the different depth at which the photons are absorbed, and the nonuniformity of the avalanche multiplication efficiency. This results in differing delays in the build-up of the carrier avalanche and in different avalanche transit times. Consequently the TTS depends on the wavelength and the voltage. Moreover, if a passive quenching circuit is used, the reverse voltage may not have completely recovered from the breakdown of the previous photon. The result is an increase of the TTS width or a shift of the TTS with the count rate. 

Compared to PMTs, single photon avalanche photodiodes (SPADs) have a considerably higher efficiency in the near infrared. Figure 6.17 compares the QE of a silicon SPAD module [408] with the QE of a GaAsP PMT [214]. 

S. Cova, S. Lacaita, M. Ghioni, G. Ripamonti, T.A. Louis, 20-ps timing resolution with single-photon avalanche photodiodes. Rev. Sci. Instrum. 60, 1104-110(1989)... 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

A. Lacaita, S. Cova and M. Ghioni, Four-hundred-picosecond single photon timing with commercially available avalanche photodiodes, Rev. Sci. Instmm. 59, 1115-1121 (1988). 

Due to the random nature of the amplification process in a photomultiplier tube or avalanche photodiode, the single-photon pulses have a considerable amplitude jitter (see Fig. 1.5). For analog processing, the amplitude jitter contributes to the noise... 

Multiphoton detection is a frequent souree of errors in attempts to use standard avalanche photodiodes as single photon deteetors. If the diode is not really operated in the breakdown region, a detectable output pulse is obtained only if several photons are detected within the impulse response time of the diode, with a similar results as shown in Fig. 7.62. 

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