Study of avalanche statistics in very low noise AlGaAsSb APDs using a multi-channel analyzer

The usefulness of avalanche photodiodes (APDs) resides in their ability to produce internal gain via impact ionization without generating excessive noise. This process is stochastic and the gain values fluctuate around a mean value, giving rise to the so-called excess noise. In this work, we evaluate the gain fluctuations in APDs using a multi-channel analyzer (MCA). Two Al0.85Ga0.15As0.56Sb 0.44APDs, one p-i-n and one n-i-p were used. Illuminated with a pulsed light source, the APDs were connected to a charge-sensitive amplifier, counting the number of charges created by each avalanche event initiated by the light pulse. The signal was subsequently sent to an MCA, recording the gain values and outputting a gain spectrum. Both APDs were investigated for mean gains up to ∼ 9. For a given mean gain, the gain distribution for the n-i-p diode was found to be significantly broader than for the p-i-n diode, as expected from the excess noise values previously measured in those devices. The coefficient of variance (CoV), defined as the ratio of standard deviation to mean value of the gain peaks, was found to be low for the p-i-n APD, consistent with the low excess noise values in this material. For higher mean gain values, the CoV of the n-i-p APD gave higher values than for the p-i-n APD, again corroborating the conventional excess noise measurements.