Experimental Determination of the Gain Distribution of an Avalanche   Photodiode at Low Gains
  A measurement system for determining the gain distributions of avalanche photodiodes (APDs) in a low gain range is presented. The system is based on an ultralow-noise charge--sensitive amplifier and detects the output carriers from an APD. The noise of the charge--sensitive amplifier is as low as 4.2 electrons at a sampling rate of 200 Hz. The gain distribution of a commercial Si APD with low average gains are presented, demonstrating the McIntyre theory in the low gain range. 
Kenji Tsujino, Makoto Akiba, and Masahide Sasaki