Excess noise figure, also called excess noise index, is a value that can be used to estimate excess noise. The formula for calculating excess noise from the excess noise figure is gain raised to the excess noise figure. Excess noise figure is an empirically measured value that allows the sensor’s user to estimate the excess noise at a given voltage or gain setting. The excess noise figure is determined by sweeping the gain setting then measuring the excess noise. The excess noise figure is then determined to generate a linear slope that approximates the slope of the excess noise vs. gain plot.
Si APDs’ excess noise is typically between 2 and 6. This is much higher than other detectors like PMTs (between 1.2 and 1.3) and MPPCs/SiPMs (between 1.1 and 1.2). One of the reasons why Si APDs’ excess noise values are relatively high is the ability for both electrons and holes to cause impact ionization with varied efficiency.
The absorption depth of the photon varies depending on the wavelength, with shorter wavelengths absorbed closer to the front surface. Where the photon is absorbed and the avalanche is initiated affects how the carrier multiplication progresses in the avalanche region. The charge carriers must also traverse a minimum distance to generate sufficient kinetic energy to initiate impact ionization. As a result, the stability of the avalanche multiplication depends on the location where the first charge carrier is generated.
Dino Butron is an Applications Engineer, specializing in all of our point detectors and signal-to-noise simulations. He and his photonic sidekick (his cat Bash) tackle technical issues daily.
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