Excess Noise Factors for Conventional and Superlattice Avalanche Photodiodes and Photomultiplier Tubes

Light falling on a photodetector produces an output current that fluctuates. The noise in this signal arises from two sources: randomness in the photon arrivals and randomness in the carrier multiplication process intrinsic to the photodetector. A general formula is derived for the variance of the photodetector output current in terms of parameters characterizing these two sources of randomness (the photon-number variance-to-mean ratio for the light and the excess noise factor for the detector). An important special case of this formula illustrates that the output-current variance is directly proportional to the detector excess noise factor when the number of photons at the input to the detector is Poisson distributed. Explicit expressions for excess noise factors are provided for three kinds of photodetectors: the double-carrier conventional avalanche photodiode, the double-carrier superlattice avalanche photodiode, and the photomultiplier tube. The results for the double-carrier superlattice device are new; it is shown that even a small amount of residual hole ionization can lead to a large excess noise factor. Comparisons are drawn among the detectors in terms of their noise properties.