Excess Noise
As mentioned above, this is the noise due to the multiplication process at a gain, M and is denoted by F(M) and can often be expressed as:
where is the ratio of the hole impact ionization rate to that of electrons. For an electron multiplication device it is given by the hole impact ionization rate divided by the electron impact ionization rate. It is desirable to have a large asymmetry between these rates to minimize F(M), since F(M) is one of the main factors that limit, among other things, the best possible energy resolution obtainable.
Read more about this topic: Avalanche Photodiode
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