The impact ionization process was studied in thin multiplication layers used in wide-bandwidth SAGM-APDs (separate absorption, grading, and multiplication avalanche photodiodes) using wave-vector-dependent threshold energies for impact ionization and a full-band Monte Carlo model. Results show that the number of ionizing collisions is reduced in thin layers, leading to an increase in the effective beta / alpha ratio and explaining the experimentally observed reduction in the excess noise factor. It is also shown that, even though the values of alpha and beta in III-V semiconductors converge at high electric fields, it is possible to use thin multiplication regions to reduce the excess noise factor and at the same time increase the bandwidth of the device operation.
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Chandramouli et al. (1993) studied this question.
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